Air/fuel ratio problem, anyone help?

[tvrlee]

I have a Chim 400, only mod is is an ACT induction kit, that i put on a dyno last October, it showed BHP at 206, torq at 260 and the AFR was bad, down to less than 10:1 at high revs.

I then changed the coolant sensor to ecu, checked the AFM and fitted an adjustable fuel pressure regulator, with some help from Richard Bradford (diycerb) we put a AFR meter on the exhaust and tried it out, it showed that with the fuel pressure set at 36psi it was still running rich, very rich. After several adjustments we managed to get it running at 14.7:1 at idle and whilst closed loop on ecu, and it went down to 12:1 when open loop, which was ok, but still a tad rich.

The problem is that the fuel regulator is set right down to 29psi to acheive this, which we think is masking an underlying problem, we put it back on the dyno and got a bhp of 212 and no change to torq, the afr as still reading rich at 4500 to 5000 rpm but not as bad as before. Weare worried about it running too lean as well by turning down the pressure.

Has anyone else come across this or know of an answer? any help would be appreciated.

Lee

[Chassis 33]

Could it be a duff injector allowing a constant flow? If that wasn't cycling properly you'd one cylinder running rich and the other three running lean on one bank as the lambda tries to compensate. Reducing fuel pressure will artificially reduce the flow to the rail and hence lean off the mixture.

Regards
Iain

[K4000JB]

Chassis 33 said:

Could it be a duff injector allowing a constant flow? If that wasn't cycling properly you'd one cylinder running rich and the other three running lean on one bank as the lambda tries to compensate. Reducing fuel pressure will artificially reduce the flow to the rail and hence lean off the mixture.

one way to check this would be to use one of those thermopoint lazer thermometers all the headers should be around the same temp any deveation would point to the odd cylinder or cylinders being the problem. you didnt say when you were checking the pressure presumably whilst the engine was idling. you should get approx 3bar(42psi) with the ignition on ,engine not running,(simulating wide open throttle)any more than this could be why you are getting your problem. whilst idling you are looking for approx 2.5bar(35psi)

i hope this helps
ant

[diycerb]

Thanks for your suggestions guys, we'll take a look.

Lee, I can get hold of a laser thermometer, I'll have a word. A rich cylinder will be detected by a lower temp than a correctly running cylinder.

Rich.

[GreenV8S]

If you've checked that the AFR is correct then I don't see why you are worrying about it going lean.

If it seems to need the 'wrong' fuel pressure to get a sensible AFR then something must be wrong. Have you checked the sensor readings at the ECU? Things like coolant temp senders are easy to test and I wouldn't be changing those unless there was an obvious fault, but equally it may be that while the sender itself is OK the ECU may not be getting the correct signal from it. Hence see what the ECU is actually seeing, and if any of the inputs seem wrong then investigate why.

How confident are you that the fuel pressure reading is correct? Are you sure the gauge is accurate and you're using it correctly?

206/112 bhp is not bad for a healthy 4 litre so I wouldn't go looking for major problems.

[rev-erend]

And don't forget to remove the Airpipe and block it on the fuel pressure thingie when you check the reading.

Have the injectors been changed at any point ?

If bigger injectors have been fitted and it's not been remapped - it would run RICH !

[K4000JB]

i was thinking more of alone the lines of finding a cylinder running cooler than the rest which would mean it isnt burning correctly e.g sticking open injector, plug or lead braking down slightly therefore giving a rich mixture. also no need to disconnect the vac pipe off the reg if you follow my directions(with engine idling you should have your lowest reading approx 2.5bar and with ignition on but engine not running ,therfore not producing a vacuum, you should see your maximum pressure approx 3.0bar).

[tvrlee]

Thanks for all the help.

Just to clarify a couple of things, the pressure is at 29psi with the engine at idle, i havent checked with the ignition on but not running yet, so will do that. I know it doesnt show everything but there is a fault code reader attached to the ECU showing no faults. I have also replaced the HT leads with a new Magnecore set. As far as i am aware the injectors have not been changed, nothing in the history anyway.

I am aware that the readings from the dyno with regard to BHP and torq are not bad for a '94 400 but the car is still dumping fuel when under load at high revs. This is not a problem when running around locally but it uses alot of fuel when opened up.

Lee

Edited by tvrlee on Friday 1st February 07:48

[GreenV8S]

tvrlee said:

Just to clarify a couple of things, the pressure is at 29psi with the engine at idle

Do you know what it should be under those conditions?

[trackcar]

I dont understand the numbers on this thread .. IMO it would be very unusual that a std 4litre would make well over 200hp with 10:1 AFR .. thats even mega rich for a marginal high hp turbo install, your car would be down around 150-160hp on that AFR if it ran at all without spluttering or at least sounding very wooly and throwing black soot everywhere.

What makes you think the AFR is really 10:1 other than the AFR gauge used in the test? Is that accurate and tested regularly for accuraccy? What do you call excessive fuel consumption under load? (if it really is 212hp its going to gobble some juice anyway even fuelled correctly). Has the fuel economy dropped off sbstantially since you've owned the car?

There's so many variables to take in that its impossible to give a fair diagnosis, you really need to get the car somewhere that it can be properly diagnosed. You maybe have a problem, you may not. If you think your car is 212ish hp then that would suggest a very healthy modded engine with no apparent serious faults .. you might be chasing something that's not really there!



My advice : get a second opinion.

[diycerb]

Hi Joo,

The first time we saw this problem was on the dyno. Its a dyno dynamic dyno, well respected company down here in the south, no real concerns there.

My LM1 AFR meter then confirmed the rich mixture above 4k rpm. The LM1 also confirms below say 3.5K the car runs perfectly showing an AFM of 14.7 (closed loop lambda).

After much checking and changing of sensors we fitted an adjustable fuel pressure reg with gauge. This was set up to the book quoted 36psi at idle. The car was then run with the LM1 and confirmed we still had 10:1 at high revs, we then reduced the pressure, about 2 psi at a time, making sure the ecu was still able to control to 14.7:1 normally. After a few runs we ended up at 30psi and an AFR of IIRC 12.5:1. We then got the car back on the dyno and this confirmed the changes we'd made, we'd picked up about 6bhp from the original run.

We certainly haven't seen clouds of black smoke and the car revs freely with no misfires. 10:1 is very rich but not rich enough to cause poor running. Anyway its about 12.5:1 now but at 29psi fuel pressure.

As previously recommended we'll have a look at the EGT's and I think it a good idea to check the sensor voltages at the ECU terminals just in case there is a high resistance wiring problem causing less voltage to be seen at the ECU.

Rich.

[blitzracing]

The ecu sets the overall fueling for the lamda control range at tickover, and it has quite a wide margin. This takes about 20 idle seconds to set, but will cover up a wide range of wrong sensor inputs, as lambda control is most important. I have tried altering sensor inputs, and the ECU simply re-adjusts the lambda control. This all goes out of the window once its open loop, as it will only have the fixed map, plus basic sensor input. For overfueling I would look at temperature sender resistance too high, or airflow meter output too high but I cant pass comment on the TPS and fuel temp, Ive not tried altering them. The highest voltage Ive seen of the AFM output is around 7.5 volts under peak RPM, full throttle on a 4.5. Difficult to measure at warp 9 on the road however! I dont know why the AFM output should be too high, unless the earth is poor to the ECU, that distorts the output.

Mark

[HarryW]

Assuming your AFR measurements are correct then there's is not a lot wrong with 212hp from a 4ltr (actually quite strong), yes its probably too rich.
If you are happy with your sensors feeding the ECU then if you have access to another Tiv then try swapping the AFM as if its over reading then the ECU is just doing its job of putting to extra fuel to match the high air load. Other than that I would take it to a garage for a diagnostic run on the rollers and get that second opinion.

[spend]

Is 12.5 AFR unusual for the TVR at the topend? I though that had been mentioned as one of its characteristics - probably from the threads of MA explaining the changes he made..

Dave

[HarryW]

blitzracing said:

The ecu sets the overall fueling for the lamda control range at tickover, and it has quite a wide margin. This takes about 20 idle seconds to set, but will cover up a wide range of wrong sensor inputs, as lambda control is most important. I have tried altering sensor inputs, and the ECU simply re-adjusts the lambda control. This all goes out of the window once its open loop, as it will only have the fixed map, plus basic sensor input. For overfueling I would look at temperature sender resistance too high, or airflow meter output too high but I cant pass comment on the TPS and fuel temp, Ive not tried altering them. The highest voltage Ive seen of the AFM output is around 7.5 volts under peak RPM, full throttle on a 4.5. Difficult to measure at warp 9 on the road however! I dont know why the AFM output should be too high, unless the earth is poor to the ECU, that distorts the output.

Mark

I used to connect a multimeter to the AFM output via flying leads to inside the cabin and do back to back testing on any improvements I could achieve with intake routing and types of piping. I never saw over 5 volts which I understood was fsd for the 5AM AFM as it would hit that around 5.5Krpm the mapping had to be extrapolated above that for extra fuel. That was a 4 ltr making around 275hp/285lbs.

[GreenV8S]

diycerb said:

After a few runs we ended up at 30psi and an AFR of IIRC 12.5:1. We then got the car back on the dyno and this confirmed the changes we'd made, we'd picked up about 6bhp from the original run.

We certainly haven't seen clouds of black smoke and the car revs freely with no misfires. 10:1 is very rich but not rich enough to cause poor running. Anyway its about 12.5:1 now but at 29psi fuel pressure.

I don't know how much experience you have of adjusting fuel pressure, but you need to be aware that the Cerbera fuel pressure is set quite differently to the Rover V8. The Cerbera (as far as I know) uses constant fuel pressure whereas the RV8 system sets the pressure relative to manifold depression. Hence you would normally set the pressure with the engine stopped or the vac reference line disconnected to avoid having any depression. If you're trying to set the fuel pressure with the engine at idle there will be a lot of manifold depression, the fuel pressure will be correspondingly lower. Typically the manifold depression will be of the order of 0.7 bar so your nominal 2.5 bar fuel pressure will reduce to 1.8 bar (around 26 psi) at idle.

Given that the engine is producing about the right power, and the AFRs are about right, and the fuel pressure is about right, I think you are chasing a non-problem.

Edited by GreenV8S on Friday 1st February 12:39

[blitzracing]

[/quote]
I used to connect a multimeter to the AFM output via flying leads to inside the cabin and do back to back testing on any improvements I could achieve with intake routing and types of piping. I never saw over 5 volts which I understood was fsd for the 5AM AFM as it would hit that around 5.5Krpm the mapping had to be extrapolated above that for extra fuel. That was a 4 ltr making around 275hp/285lbs.
[/quote]

Its possible I was looking at overshoot on my analogue meter, as Like you I was in the passanger seat with wires from the AFM, and running out of road. As a matter of interest a hairdryer on cold makes a pretty good test for checking the rise of the AFM output, without endangering one self.

[diycerb]

GreenV8S said:

I don't know how much experience you have of adjusting fuel pressure, but you need to be aware that the Cerbera fuel pressure is set quite differently to the Rover V8. The Cerbera (as far as I know) uses constant fuel pressure whereas the RV8 system sets the pressure relative to manifold depression. Hence you would normally set the pressure with the engine stopped or the vac reference line disconnected to avoid having any depression. If you're trying to set the fuel pressure with the engine at idle there will be a lot of manifold depression, the fuel pressure will be correspondingly lower. Typically the manifold depression will be of the order of 0.7 bar so your nominal 2.5 bar fuel pressure will reduce to 1.8 bar (around 26 psi) at idle.

Given that the engine is producing about the right power, and the AFRs are about right, and the fuel pressure is about right, I think you are chasing a non-problem.

Hi Peter, thanks for that explaination. I know of the difference between the the fuel setups but maybe we set the stqaic pressure incorrectly.

The first rolling road and my AFR log showed we had a problem, 10:1 above 4K rpm. Not able to find fault with anything, we suspected and changed the fuel pressure reg. We may have set it incorrectly high, and therefore replicated the problem with the original reg. We've then lowered the fuel pressure, very worried we were treating the syptoms rather than curing the fault.

The fault being the original pressure reg and now user error!!

Rich