Manifold Pressure
Discussion
You need to measure it, each car / cam will differ. You can't effectively measure the fuel pressure without simultaneously measuring the vac and calculating the difference.
Essentially the engine is sucking hard with closed throttle at idle, so to achieve 3 bar across the injector, you may only need 1 bar (vs atmosphere) in the fuel rail.... which is the core reason for the FPR vac hose.
Essentially the engine is sucking hard with closed throttle at idle, so to achieve 3 bar across the injector, you may only need 1 bar (vs atmosphere) in the fuel rail.... which is the core reason for the FPR vac hose.
spend said:
You need to measure it, each car / cam will differ. You can't effectively measure the fuel pressure without simultaneously measuring the vac and calculating the difference.
Essentially the engine is sucking hard with closed throttle at idle, so to achieve 3 bar across the injector, you may only need 1 bar (vs atmosphere) in the fuel rail.... which is the core reason for the FPR vac hose.
Thanks, I appreciate all of that & am very aware of the relationship between throttle bitterly position & manifold depression.Essentially the engine is sucking hard with closed throttle at idle, so to achieve 3 bar across the injector, you may only need 1 bar (vs atmosphere) in the fuel rail.... which is the core reason for the FPR vac hose.
I freely admit it's a long time since I studied all this at college, but I think we covered that in week one
What I don't have is some general expected figures for the RV8, so all I'm really asking/looking for is a general figure.
Clearly there will be a specification/operating range for the fuel pressure regulator, I would also be interested to know this.
I was testing manifold depression yesterday & plotting it's relationship to fuel pressure at the rail.
I was also simulating the effects of artificially increasing the depression at the FPR, & plotting its effects on fuel pressure & AFR at idle.
Last night I concocted an improved temporary fuel return bypass system I could control with a tap.
This allowed me to switch from the standard return line to a shorter, lower & less restrictive return simply by opening & closing a tap.
With the tap closed the fuel was directed to the tank in it's normal path, but with the tap opened the returned fuel from the rail was allowed to pass unhindered to a jerry can placed at the side of the car.
Turning the tap on and off while the car was idling & exhibiting the condition made no difference to the fuel pressure & excessively rich state.
In the next test, I closed the tap as it seemed to make little or no difference, then artificially applied an increasing level of vacuum to the FPR while observing the fuel pressure, idle quality and mixture.
As the vacuum was increased beyond a certain level the car's idle, fuel pressure & mixture all came good
Clearly opening the return port more on the FPR than my current manifold depression can muster is all it needs to solve the problem.
My diagnostic tests clearly show the rich petrol idle condition is actually not a result of a return restriction as I had first suspected, but is a direct result of insufficient depression acting on the FPR.
All this is just basic diagnostics really & certainly nothing ground braking, but it is extremely conclusive all the same.
As we know there are a number of potential reasons for insufficient manifold depression, but as the condition is only really appears after the car has returned from a long hot run I am inclined to assume I have an induction leak.
But as any good mechanic will tell you assumption is the mother all all ffluck ups.
The truth is low manifold depression can be the result of many different things; cam timing, ignition timing, manifold leak, valve train wear, rings, exhaust restriction, collapsed vacuum line/s, faulty brake servo, faulty MAP sensor, faulty FPR, ect ect ect.
I was always taught to work with known figures & challenge my own diagnosis by applying more than one test referencing these figures.
I was also taught conclusive diagnosis can only be found when the results of the tests (plural) point to the same conclusion.
That's why I'd like to understand what vacuum figures people typically see at their Chimaera plenum, this will be an extremely useful reference point for me
As will understanding the figures/working range of the standard fuel pressure regulator.
Perhaps someone can help?
If its any use Dave my brothers stock 400 runs at around 48 KPA at idle 850ish my 400 having the lumpier/HC cam is around 55 KPA 950ish personally I don't think its a Vacuum problem you have 
unplugging the FPR hose will not make the car run so rich it cuts your eyes 
are you sure the IAT sensor is not faulty as when these go open circuit the fuelling can go through the roof 
plug in the Lappy to see what the gauges are telling you 
better still run a Datalog session its why we have all this fancy-Nancy ECU stuff for
personally I don't think its a Vacuum problem you have unplugging the FPR hose will not make the car run so rich it cuts your eyes are you sure the IAT sensor is not faulty as when these go open circuit the fuelling can go through the roof plug in the Lappy to see what the gauges are telling you better still run a Datalog session its why we have all this fancy-Nancy ECU stuff for Edited by Sardonicus on Tuesday 23 July 12:08
Sardonicus said:
If its any use Dave my brothers stock 400 runs at around 48 KPA at idle 850ish my 400 having the lumpier/HC cam is around 55 KPA 950ish personally I don't think its a Vacuum problem you have unplugging the FPR hose will not make the car run so rich it cuts your eyes are you sure the IAT sensor is not faulty as when these go open circuit the fuelling can go through the roof plug in the Lappy to see what the gauges are telling you better still run a Datalog session its why we have all this fancy-Nancy ECU stuff for
Thanks Simon, an excellent response as always, much appreciated personally I don't think its a Vacuum problem you have unplugging the FPR hose will not make the car run so rich it cuts your eyes are you sure the IAT sensor is not faulty as when these go open circuit the fuelling can go through the roof plug in the Lappy to see what the gauges are telling you better still run a Datalog session its why we have all this fancy-Nancy ECU stuff for Edited by Sardonicus on Tuesday 23 July 12:08
Apologies, my brain & gauges are all from when I used to do a bit of diagnostic work on Noah's Ark.
I'm so stuck in the past I keep forgetting I've got the most sophisticated diagnostic tool already installed.
Note to self, plug the lapadoodle in
So what have I discovered so far?
1. It goes rich when it gets proper hot after a long run but only shows up at idle
2. I can elevate the condition by applying 20hg" to the FPR
But if honest with myself that's not conclusive evidence of an induction leak in itself.
I have a Stealth cam, so your figure of 55 fruit pastels at 950ish (throttle butterfly closed) is what I'll run with, I make it about 17Hg" in old money.
But I'll take your advice & put my 1930's barometer back in it's wooden box, break out the lapadoodle & start looking at fruit pastels on the screen.
I guess by pulling on the FPR with my vac tester its possible I could just be masking the real issue
Tell me more about air temp sensors Simon
- What air temps am I looking for?
- What are the other symptoms of a faulty IAT sensor?
- It's just a resistor right, other than viewing/logging the temp figures on the lapadoodle can you give me some figures/pointers so I can get my old school meter on the little feckkker?
- What makes you so sure it's the IAT sensor?
If I read correctly all works fine when he connects a test hose to the fuel return? That would indicate to me either restricted fuel return or pressurized tank (not breathing out properly?).
The IAT sensor is sensitive, I go to extreme lengths building tufnol holder that clips into pipe by air filter to hold the IAT and even then ensure it faces away from the rad. Still can't see it causing "eye stinging" symptoms even under most extreme circumstances unless something else is grossly messed up?
Smacks of leaks to me TBH.
The IAT sensor is sensitive, I go to extreme lengths building tufnol holder that clips into pipe by air filter to hold the IAT and even then ensure it faces away from the rad. Still can't see it causing "eye stinging" symptoms even under most extreme circumstances unless something else is grossly messed up?
Smacks of leaks to me TBH.
I get a lot of heat soak when stationary but that would show up as a lean condition normally so check your air temp compensation table just in case it has been told to dump loads of fuel in as the temps get higher( should actually reduce in the real world as the less air molecules means the less fuel you need as you know) but I add fuel to mine to keep the idle running at 13.7 all the way up the temp scale,
If it was the IAT sensor at fault then it would run rough as a dog not just at idle,
If it was the IAT sensor at fault then it would run rough as a dog not just at idle,
spend said:
If I read correctly all works fine when he connects a test hose to the fuel return? That would indicate to me either restricted fuel return or pressurized tank (not breathing out properly?).
The IAT sensor is sensitive, I go to extreme lengths building tufnol holder that clips into pipe by air filter to hold the IAT and even then ensure it faces away from the rad. Still can't see it causing "eye stinging" symptoms even under most extreme circumstances unless something else is grossly messed up?
Smacks of leaks to me TBH.
Well that's what I thought, but I'm open to hear what Simon has to say re the air temp sensor.The IAT sensor is sensitive, I go to extreme lengths building tufnol holder that clips into pipe by air filter to hold the IAT and even then ensure it faces away from the rad. Still can't see it causing "eye stinging" symptoms even under most extreme circumstances unless something else is grossly messed up?
Smacks of leaks to me TBH.
In my test yesterday I gave the return a new completely free unhindered path into a jerry can sat on the floor next to the car.
Switching back & forth from this to the current tank return using my tap arrangement showed no real improvement in the rich idle issue.
This to me is pretty conclusive evidence it's not a return line restriction.
Unless you count the return port of the FPR as a restriction, because one thing is for sure....
If you apply 20Hg" to the FPR using my vac tester the issue goes away, if you then bleed off that vacuum the condition returns at around 15Hg" and gets very bad below that... ending in a stall at 10Hg".
As I'm sure the old guard know already, there are lots of revealing tests you can do with an old school vac gauge, reading the gauge properly can tell you a lot about what's really going on.
http://www.gregsengine.com/using-a-vacuum-gauge.ht...
Use one in conjunction with a fuel pressure gauge and it should be possible to pinpoint & prove a huge number of mechanical or other issues in a fuel injected engine.
I'm with you Spend, right now everything seems to point to there being insufficient vacuum in my plenum at idle to open the return port in the FPR enough.
The result is excessive fuel pressure which in turn means the injectors are being overcome, which means things go proper rich.
Insufficient depression in the plenum either means I'm losing it somewhere, or I have ring, valve guide or valve issues.
Ok so I haven't done a bleed down compression test yet, but she uses very little oil & consistently puts down a healthy 260 ponies on the rollers.
I should say at this point the eye sting effects are only really evident when backing in to my little lockup, space is very tight so the effects are extremely strong on your eyes & thought.
But trust me when I say you don't need an AFR reading or an exhaust gas analyser to know it's running super rich, even outside.
I'm interested to understand the air temp theory but remember this is a Canems Dual Fuel system.
That means both fuels share that same air temp sensor, to you'd think I'd see the same issues on gas, which I don't!
Although the vaporiser is (amongst other things) just a giant FPR, I doesn't need to reference manifold depression to function correctly.
To my mind this is all further evidence pointing to a vacuum leak when things get hot, rather than the air temp sensor failing.
But I remain open minded.
Thanks for everyone's input, it's only a little issue for me TBH as I'm sniffin the gas 95% of the time.
But I would like to get it sorted in the next week or so before my big trip.
Cheers chaps
Edited by ChimpOnGas on Tuesday 23 July 14:30
Discopotatoes said:
I would say check your maps in the canems in case there's anything amiss there on the petrol side,
Could it possibly be then canems unit itself?
When did it show up as a fault and was it running fine before the conversion?
Could it possibly be then canems unit itself?
When did it show up as a fault and was it running fine before the conversion?
Edited by Discopotatoes on Tuesday 23 July 14:39
What's the lappy gauges say Dave? thing is if the IAT is the issue how is he going to know if it not over-fuelling through out the rev range unless he is using a WB sensor or running behind 
over-fulling is always more of an issue at idle speeds than upper RPM anyway IMO, Dave maybe worth having a natter with the Canem boys might be something they have seen before like always problems like this can be hard to diagnosis unless your with the patient could even be a MAP sensor failure through getting heat-soak of the nearby ex manifold .............. GET THE LAPTOP ON IT 
all maybe revealed Edited by Sardonicus on Tuesday 23 July 14:51
Sardonicus said:
What does it do on LPG?
Another good question, answered above "Ol Gasbag" is now doing 25 miles to the gallon on LPG, which is a 45mpg equivalent.
She makes 260hp on both fuels, and you 100% cant tell the difference between the two.
She drives great guns on petrol, and just the same on gas.
My mad French car collecting mate is staying with us for a few days, as an owner of both a 4.0 litre and a mighty 5.0 litre Chimaera (along with 70 other performance oddities) he was very interested to sample the conversion himself.
So last night I took him for a one junction test around the top half of the M25.
In both tests I gave her a proper spanking for being naughty at idle on petrol
One way we went on petrol, & the other on LPG.
On petrol he said it's somewhere between his 4.0 litre & 5.0litre.
On the return I switched to gas... this time my sceptical French friend said nothing.
He just roared with uncontrollable laughter
When we slowed he finally recovered from his uncontrollable hysterics & managed to muster.....
"GPL, so it's not just for economy then"
Honestly folks it's an absolute riot on BOTH fuels, I now just need her idling nicely on petrol on hot days
It's not really a big issue in the great scheme of things
ChimpOnGas said:
Another good question, answered above
"Ol Gasbag" is now doing 25 miles to the gallon on LPG, which is a 45mpg equivalent.
She makes 260hp on both fuels, and you 100% cant tell the difference between the two.
She drives great guns on petrol, and just the same on gas.
My mad French car collecting mate is staying with us for a few days, as an owner of both a 4.0 litre and a mighty 5.0 litre Chimaera (along with 70 other performance oddities) he was very interested to sample the conversion himself.
So last night I took him for a one junction test around the top half of the M25.
In both tests I gave her a proper spanking for being naughty at idle on petrol
One way we went on petrol, & the other on LPG.
On petrol he said it's somewhere between his 4.0 litre & 5.0litre.
On the return I switched to gas... this time my sceptical French friend said nothing.
He just roared with uncontrollable laughter
When we slowed he finally recovered from his uncontrollable hysterics & managed to muster.....
"GPL, so it's not just for economy then"
Honestly folks it's an absolute riot on BOTH fuels, I now just need her idling nicely on petrol on hot days
It's not really a big issue in the great scheme of things
See my EDITED post above Dave, no I meant what is the idle behaviour like on the GAS?"Ol Gasbag" is now doing 25 miles to the gallon on LPG, which is a 45mpg equivalent.
She makes 260hp on both fuels, and you 100% cant tell the difference between the two.
She drives great guns on petrol, and just the same on gas.
My mad French car collecting mate is staying with us for a few days, as an owner of both a 4.0 litre and a mighty 5.0 litre Chimaera (along with 70 other performance oddities) he was very interested to sample the conversion himself.
So last night I took him for a one junction test around the top half of the M25.
In both tests I gave her a proper spanking for being naughty at idle on petrol
One way we went on petrol, & the other on LPG.
On petrol he said it's somewhere between his 4.0 litre & 5.0litre.
On the return I switched to gas... this time my sceptical French friend said nothing.
He just roared with uncontrollable laughter
When we slowed he finally recovered from his uncontrollable hysterics & managed to muster.....
"GPL, so it's not just for economy then"
Honestly folks it's an absolute riot on BOTH fuels, I now just need her idling nicely on petrol on hot days
It's not really a big issue in the great scheme of things
Sardonicus said:
ee my EDITED post above Dave, no I meant what is the idle behaviour like on the GAS?
Sorry Simon, I went off on one again Idles good mate, well saying that I'm now wondering if I just need to flick it onto gas when it's doing the rich idle on petrol thing.
Because even on petrol it doesn't do it until it gets good & hot after a run.
I'll test it so I can give you a conclusive & confirmed answer.
Actually when it does it most, is when I flick to petrol after a good run on LPG.
This got me thinking
On my system (as with most LPG conversions) the fuel pump remains running even when you're on gas.
There are some pretty sound reasons for this, amongst others is ensures the fuel in the rail doesn't boil & changing back to petrol is seamless.
Obviously as part of all this the FPR is operating as normal & manifold depressions acting on it remain the same too.
So there the FPR is fluttering away as I happily drive along on gas, it continues spilling varying volumes of petrol back into the tank depending on how much I open the throttle & how much load the engine is under.
The only thing that changes is the petrol injectors aren't being fired.
So if the FPR is designed to bleed petrol back to the tank at a rate that's been configured to include the volume of fuel used when running on petrol so maintaining the all important 36/37psi rail pressure.
Could it be when there's no fuel running through the injectors the system reaches critically mass & overly pressurises the rail?
Could this be why I'm seeing a rise in rail pressure why it goes rich on petrol after switching from gas?
Flick from LPG to petrol at idle after a long hot run and the now opening injectors are immediately overcome by all that pressure they were never designed to deal with, thus creating the super rich petrol idle condition?
Just a thought?
Probably a red herring too, but I see no harm in throwing the idea out there.
I'll double check the LPG idle quality, all I can conclusively say at this point is it never stalls on gas, even when properly hot.
Lapy-top-icus at the ready Simon 
But I still can't find my bob weights & springs draw, let alone my needle & jet set?
Note to self - "Must learn Canems Injection Tuner software
I'm getting some lessons off the Lloyd boys before I fek it up Silicone Kid styleeee
But I still can't find my bob weights & springs draw, let alone my needle & jet set?
Note to self - "Must learn Canems Injection Tuner software
I'm getting some lessons off the Lloyd boys before I fek it up Silicone Kid styleeee
Edited by ChimpOnGas on Tuesday 23 July 16:23
ChimpOnGas said:
Sorry Simon, I went off on one again
Idles good mate, well saying that I'm now wondering if I just need to flick it onto gas when it's doing the rich idle on petrol thing.
Because even on petrol it doesn't do it until it gets good & hot after a run.
I'll test it so I can give you a conclusive & confirmed answer.
Actually when it does it most, is when I flick to petrol after a good run on LPG.
This got me thinking
On my system (as with most LPG conversions) the fuel pump remains running even when you're on gas.
There are some pretty sound reasons for this, amongst others is ensures the fuel in the rail doesn't boil & changing back to petrol is seamless.
Obviously as part of all this the FPR is operating as normal & manifold depressions acting on it remain the same too.
So there the FPR is fluttering away as I happily drive along on gas, it continues spilling varying volumes of petrol back into the tank depending on how much I open the throttle & how much load the engine is under.
The only thing that changes is the petrol injectors aren't being fired.
So if the FPR is designed to bleed petrol back to the tank at a rate that's been configured to include the volume of fuel used when running on petrol so maintaining the all important 36/37psi rail pressure.
Could it be when there's no fuel running through the injectors the system reaches critically mass & overly pressurises the rail?
Could this be why I'm seeing a rise in rail pressure why it goes rich on petrol after switching from gas?
Flick from LPG to petrol at idle after a long hot run and the now opening injectors are immediately overcome by all that pressure they were never designed to deal with, thus creating the super rich petrol idle condition?
Just a thought?
Probably a red herring too, but I see no harm in throwing the idea out there.
I'll double check the LPG idle quality, all I can conclusively say at this point is it never stalls on gas, even when properly hot.
Its a good point Dave, but surely once the injectors start opening again, fuel rail pressure will go to normal (IF its over pressurising) in a short period of time. I know it eventually stalls but how long will it continue to run rich at idle for, or is it pretty permanent.Idles good mate, well saying that I'm now wondering if I just need to flick it onto gas when it's doing the rich idle on petrol thing.
Because even on petrol it doesn't do it until it gets good & hot after a run.
I'll test it so I can give you a conclusive & confirmed answer.
Actually when it does it most, is when I flick to petrol after a good run on LPG.
This got me thinking
On my system (as with most LPG conversions) the fuel pump remains running even when you're on gas.
There are some pretty sound reasons for this, amongst others is ensures the fuel in the rail doesn't boil & changing back to petrol is seamless.
Obviously as part of all this the FPR is operating as normal & manifold depressions acting on it remain the same too.
So there the FPR is fluttering away as I happily drive along on gas, it continues spilling varying volumes of petrol back into the tank depending on how much I open the throttle & how much load the engine is under.
The only thing that changes is the petrol injectors aren't being fired.
So if the FPR is designed to bleed petrol back to the tank at a rate that's been configured to include the volume of fuel used when running on petrol so maintaining the all important 36/37psi rail pressure.
Could it be when there's no fuel running through the injectors the system reaches critically mass & overly pressurises the rail?
Could this be why I'm seeing a rise in rail pressure why it goes rich on petrol after switching from gas?
Flick from LPG to petrol at idle after a long hot run and the now opening injectors are immediately overcome by all that pressure they were never designed to deal with, thus creating the super rich petrol idle condition?
Just a thought?
Probably a red herring too, but I see no harm in throwing the idea out there.
I'll double check the LPG idle quality, all I can conclusively say at this point is it never stalls on gas, even when properly hot.
carsy said:
Its a good point Dave, but surely once the injectors start opening again, fuel rail pressure will go to normal (IF its over pressurising) in a short period of time. I know it eventually stalls but how long will it continue to run rich at idle for, or is it pretty permanent.
Just discussed this with my French ECU wing man, he reminded me the condition came 3-4 minutes after change over.If shutting off the injectors on LPG raised the pressure that much, you'd expect the issue to appear immediately on change over at idle, & it doesn't.
So I think this one is a red herring.
He also said air temps were normal throughout our run, but there is a suggestion being discussed over a cup of tea right now that says we should disable the air temp sensor just to check it's influence.
Finally it's worth telling everyone the only real change to the original petrol map following the LPG conversion was a move away from full idle control valve operation to only allowing idle control during warm up.
I was happy with this change as controlling the idle after warm up exclusively from the base idle screw seemed to make it even more stable.
But it does make me wonder if the car had always had a stalling issue due to the going rich at a hot idle, it's just that the idle control valve masked the stalling element.
It's only now the idle (after warm up) is rigidly nailed to the base idle setting that the issue has been revealed.
Your thoughts on this one are welcomed
Car runs on LPG and is up to temp. Switch over to petrol with car up to temp and everything is fine for the first few minutes and then it starts. If the idle valve is now disconnected because the car is up to temp it cant be anything to do with that otherwise it would stall immediately you switched over to petrol.
Air temp sensor and heat soak, possible, but mine is positioned where yours is ex afm position, not ideal i know but i do not get any such problems. As someone mentioned earlier on have you checked the correction table for the ATS just to make sure it is not incorrectly set to throw in more fuel the hotter it gets.
Interesting one, I will keep thinking
Air temp sensor and heat soak, possible, but mine is positioned where yours is ex afm position, not ideal i know but i do not get any such problems. As someone mentioned earlier on have you checked the correction table for the ATS just to make sure it is not incorrectly set to throw in more fuel the hotter it gets.
Interesting one, I will keep thinking
Gassing Station | Chimaera | Top of Page | What's New | My Stuff