Pumaracing ? causes for low engine vacuum
Discussion
Very troublesome 03 STI.
95% of the time it runs perfect now, after a myriad of problems have been rectified though.
Randomly at cold start it has very little vacuum and will barely drive. Occasionally not starting at all. Then it can be a nightmare to start. It doesnt wet the plugs or foul them. IJPW's are all as expected etc.
Often can appear lean, although HCC go high too briefly for no apparent reason. Ignition system is 100% ( sec hand coils and new coils etc etc ) and despite the high HCC, it doesnt appear to missfire.
Once beyond that, its fine.
What can cause low engine vacuum when cold ? Idle speed is mostly normal. Electrically, everything is 100%, checked, double checked, triple checked, checked until the cows come home. Even to extent of trying multiple ecu's ( aftermarket ecu does mask the problem better than the OE ecu, but there is still a problem )
Sensor feedback is all normal, VVC working 100% etc
Mechanically is likewise. Compression, Leakdown, everything says the engine is in perfect condition and when it is running fine, it really runs sweet !!
Except when cold...it isnt sweet !
edit. by low vacuum, i mean poor...very little.
95% of the time it runs perfect now, after a myriad of problems have been rectified though.
Randomly at cold start it has very little vacuum and will barely drive. Occasionally not starting at all. Then it can be a nightmare to start. It doesnt wet the plugs or foul them. IJPW's are all as expected etc.
Often can appear lean, although HCC go high too briefly for no apparent reason. Ignition system is 100% ( sec hand coils and new coils etc etc ) and despite the high HCC, it doesnt appear to missfire.
Once beyond that, its fine.
What can cause low engine vacuum when cold ? Idle speed is mostly normal. Electrically, everything is 100%, checked, double checked, triple checked, checked until the cows come home. Even to extent of trying multiple ecu's ( aftermarket ecu does mask the problem better than the OE ecu, but there is still a problem )
Sensor feedback is all normal, VVC working 100% etc
Mechanically is likewise. Compression, Leakdown, everything says the engine is in perfect condition and when it is running fine, it really runs sweet !!
Except when cold...it isnt sweet !
edit. by low vacuum, i mean poor...very little.
Edited by stevieturbo on Monday 8th November 23:04
stevieturbo said:
Very troublesome 03 STI.
95% of the time it runs perfect now, after a myriad of problems have been rectified though.
Randomly at cold start it has very little vacuum and will barely drive. Occasionally not starting at all. Then it can be a nightmare to start. It doesnt wet the plugs or foul them. IJPW's are all as expected etc.
Often can appear lean, although HCC go high too briefly for no apparent reason. Ignition system is 100% ( sec hand coils and new coils etc etc ) and despite the high HCC, it doesnt appear to missfire.
Once beyond that, its fine.
What can cause low engine vacuum when cold ? Idle speed is mostly normal. Electrically, everything is 100%, checked, double checked, triple checked, checked until the cows come home. Even to extent of trying multiple ecu's ( aftermarket ecu does mask the problem better than the OE ecu, but there is still a problem )
Sensor feedback is all normal, VVC working 100% etc
Mechanically is likewise. Compression, Leakdown, everything says the engine is in perfect condition and when it is running fine, it really runs sweet !!
Except when cold...it isnt sweet !
03 STI? IJPW? Too many abbreviations I don't understand. Googling tells me an 03 STI is some sort of Subaru which I gather is a Japanese car of some kind but fails to disclose what an IJPW might be. I might indeed design engines but perhaps strangely to some have almost zero knowledge of or interest in the cars they might be fitted to.95% of the time it runs perfect now, after a myriad of problems have been rectified though.
Randomly at cold start it has very little vacuum and will barely drive. Occasionally not starting at all. Then it can be a nightmare to start. It doesnt wet the plugs or foul them. IJPW's are all as expected etc.
Often can appear lean, although HCC go high too briefly for no apparent reason. Ignition system is 100% ( sec hand coils and new coils etc etc ) and despite the high HCC, it doesnt appear to missfire.
Once beyond that, its fine.
What can cause low engine vacuum when cold ? Idle speed is mostly normal. Electrically, everything is 100%, checked, double checked, triple checked, checked until the cows come home. Even to extent of trying multiple ecu's ( aftermarket ecu does mask the problem better than the OE ecu, but there is still a problem )
Sensor feedback is all normal, VVC working 100% etc
Mechanically is likewise. Compression, Leakdown, everything says the engine is in perfect condition and when it is running fine, it really runs sweet !!
Except when cold...it isnt sweet !
However. Lack of vacuuum, which I take to mean induction system depression, is a mechanical issue. Sticking valves, hydraulic lifters pumping up because the oil is too thick and cold oil pressure too high would be my first thoughts. Early Ford Zetecs had such a problem and I've seen it on Ford CVHs with unduly thick oil being used. I know this ought to show up on a compression or leakdown test but perhaps those were done when the problem was not manifesting.
Injector Pulse Width
Yes, Subaru. Vacuum taken from intake manifold. Car runs solid lifters, no hydraulics.
Whilst vac is low, it is perfectly stable.
Hot or cold, compression and leakdown show a perfect engine. Tested multiple times ( this has been an ongoing nightmare for some months now )
Should also note, it is absolutely a 100% standard car.
Yes, Subaru. Vacuum taken from intake manifold. Car runs solid lifters, no hydraulics.
Whilst vac is low, it is perfectly stable.
Hot or cold, compression and leakdown show a perfect engine. Tested multiple times ( this has been an ongoing nightmare for some months now )
Should also note, it is absolutely a 100% standard car.
Edited by stevieturbo on Monday 8th November 23:20
If the tappets are mechanical and the valves are not sticking then I have to conclude it's a cam timing issue. You say it has VVC which I assume is a variable valve timing system. You say this is working ok but logic says it's not. Such a system stuck temporarily in a high rpm mode would cause poor running and low vacuum at low rpm.
Pumaracing said:
If the tappets are mechanical and the valves are not sticking then I have to conclude it's a cam timing issue. You say it has VVC which I assume is a variable valve timing system. You say this is working ok but logic says it's not. Such a system stuck temporarily in a high rpm mode would cause poor running and low vacuum at low rpm.
Both intake cams that have VVC have feedback to indicate their position. At all times with both factory ecu and aftermarket feedback indicates they are working as requested. Solenoids controlling this have also been replaced to no effect.Even deliberately targetting anywhere from 0 through to 20deg at idle, makes little difference to the problem.
TBH, Logic has gone out the window with this car, because everything says it should run fine, except it doesnt.
And pulling and stripping the engine seems like a totally pointless measure based on current tests.
Although virtually everything external to the engine has been either swapped or replaced, despite not necessarily needing to be replaced.
I know its hard to put it across on a forum, but it is very strange.
I am a mere plebian compared to some here, but could the cause & effect be the other way round? I.e. it's got low vacuum because something is making it run badly, its not running badly because of low vacuum.
If the ECU readings look correct maybe it's something that the ECU doesn't log, like fuel pressure, failing pump or regulator something?
If the ECU readings look correct maybe it's something that the ECU doesn't log, like fuel pressure, failing pump or regulator something?
I hope you get an answer to this as my Fiat Coupe has a virtually identical problem which also only manifests itself with a cold engine, the colder the engine the worse it is. I'm a bit suspicious of the variable cam timing as I have pretty much been through the same process of elimination as you.
Does the Subaru have the same kind of basic VVT with a variator/dephaser pulley that simply adjusts the inlet cam timing? I'm wondering if (for whatever reason) it gets stuck at one end of it's range when cold even though the solenoid valve appears to be working.
Does the Subaru have the same kind of basic VVT with a variator/dephaser pulley that simply adjusts the inlet cam timing? I'm wondering if (for whatever reason) it gets stuck at one end of it's range when cold even though the solenoid valve appears to be working.
Stumped is an understatement, hence I'm asking here. It's been a work in no progress for several months now, and will never pay time spent etc.
But we refuse to be beaten.
We've even went to the stupid extent of swapping parts for no logical reason from a known good vehicle.
In essence...the only part that hasnt been changed, aside from the wiring loom, is the engine unit itself.
The VVC is controlled by oil flow via a solenoid. But it is fully adjustable from 0 to about 25 degrees on this age.
It has sensor feedback for each cam, so its position can be verified.
Not 100% sure what oil is in it at present.
The origins of the problem are strange. The owner drove the car for some time with the CEL on, although at that stage it did appear ok..so he kept driving.
Then eventually it started to run bad, then stop.
But there have been so many issues along the way, its hard to know where its at now. But in theory, it should be 100%
But we refuse to be beaten.
We've even went to the stupid extent of swapping parts for no logical reason from a known good vehicle.
In essence...the only part that hasnt been changed, aside from the wiring loom, is the engine unit itself.
The VVC is controlled by oil flow via a solenoid. But it is fully adjustable from 0 to about 25 degrees on this age.
It has sensor feedback for each cam, so its position can be verified.
Not 100% sure what oil is in it at present.
The origins of the problem are strange. The owner drove the car for some time with the CEL on, although at that stage it did appear ok..so he kept driving.
Then eventually it started to run bad, then stop.
But there have been so many issues along the way, its hard to know where its at now. But in theory, it should be 100%
so back to the cause/effect scenario.
Assuming engine speed doesnt change, vacuum should be the same, regardless of if 1 cyl is firing, or all 4 ?
The only thing that will effect intake vacuum, is either sticking valves or blocked exhaust ? or blocked intake etc ?
So the poor vac reading IS a symptom of the actual problem, rather than a side effect ?
Assuming engine speed doesnt change, vacuum should be the same, regardless of if 1 cyl is firing, or all 4 ?
The only thing that will effect intake vacuum, is either sticking valves or blocked exhaust ? or blocked intake etc ?
So the poor vac reading IS a symptom of the actual problem, rather than a side effect ?
ringram said:
PCV? Dodgy vacuum lines?
petrol evap? EGR or similar etc?
Must be something feeding into the manifold..
No EGR. PCV fine, and also tested with it blocked up, along with servo etc.petrol evap? EGR or similar etc?
Must be something feeding into the manifold..
All hoses 100%
When I say low vacuum too....it really is nearly at atmospheric !!! which is baffling when the engine is actually still turning at 900rpm !!
if the engine speed is not changing during a "fault event" then the torque produced must be constant and be equal to the engines friction and parasitic losses, OR the engines friction and losses have to be increasing dramatically, until effectively WOT is required to maintain the engines rpm.
For the manifold vaccumm to decrease (pressure gets closer to atmospheric) either 1) the thottle is opening, or 2) the engine volumetric efficiency is falling. I would expect to see a higher MAP with a cold engine as the engines frictions in higher some more IMEP is required.
I have once had a crank sensor fault (was actually a damaged sensor ring tooth), that meant the ignition was ocasionally retarding (although of course the ems "showed" the incorrect value on screen) to such a degree that the engines BMEP fell so much that the throttle system had to open to eWOT (effective WOT, which at low rpm is very small angle) to maintain engine speed.
so some questions to think about:
Can you see what the throttle and ignition angle are doing during these events? (can you get a timing lamp onto the engine to see "True" ignition angle
how does the engine sound during such an event, does it sound like it is misfiring?
what sensor is telling you that the vaccumm is low and have you backed this up via a totaly seperate device?
For the manifold vaccumm to decrease (pressure gets closer to atmospheric) either 1) the thottle is opening, or 2) the engine volumetric efficiency is falling. I would expect to see a higher MAP with a cold engine as the engines frictions in higher some more IMEP is required.
I have once had a crank sensor fault (was actually a damaged sensor ring tooth), that meant the ignition was ocasionally retarding (although of course the ems "showed" the incorrect value on screen) to such a degree that the engines BMEP fell so much that the throttle system had to open to eWOT (effective WOT, which at low rpm is very small angle) to maintain engine speed.
so some questions to think about:
Can you see what the throttle and ignition angle are doing during these events? (can you get a timing lamp onto the engine to see "True" ignition angle
how does the engine sound during such an event, does it sound like it is misfiring?
what sensor is telling you that the vaccumm is low and have you backed this up via a totaly seperate device?
stevieturbo said:
so back to the cause/effect scenario.
Assuming engine speed doesnt change, vacuum should be the same, regardless of if 1 cyl is firing, or all 4 ?
The only thing that will effect intake vacuum, is either sticking valves or blocked exhaust ? or blocked intake etc ?
So the poor vac reading IS a symptom of the actual problem, rather than a side effect ?
Poor manifold vacuum isn't just a mechanical problem.Assuming engine speed doesnt change, vacuum should be the same, regardless of if 1 cyl is firing, or all 4 ?
The only thing that will effect intake vacuum, is either sticking valves or blocked exhaust ? or blocked intake etc ?
So the poor vac reading IS a symptom of the actual problem, rather than a side effect ?
The vacuum is caused by the resistance of the throttle plate/butterfly to the piston induction stroke and any problem that slows down or causes incomplete combustion on the previous power stroke will reduce the vacuum (varying ignition the timing has a large effect on vacuum).
What's the full history on this and are there/have there been any fault codes?
Max_Torque said:
if the engine speed is not changing during a "fault event" then the torque produced must be constant and be equal to the engines friction and parasitic losses, OR the engines friction and losses have to be increasing dramatically, until effectively WOT is required to maintain the engines rpm.
For the manifold vaccumm to decrease (pressure gets closer to atmospheric) either 1) the thottle is opening, or 2) the engine volumetric efficiency is falling. I would expect to see a higher MAP with a cold engine as the engines frictions in higher some more IMEP is required.
I have once had a crank sensor fault (was actually a damaged sensor ring tooth), that meant the ignition was ocasionally retarding (although of course the ems "showed" the incorrect value on screen) to such a degree that the engines BMEP fell so much that the throttle system had to open to eWOT (effective WOT, which at low rpm is very small angle) to maintain engine speed.
so some questions to think about:
Can you see what the throttle and ignition angle are doing during these events? (can you get a timing lamp onto the engine to see "True" ignition angle
how does the engine sound during such an event, does it sound like it is misfiring?
what sensor is telling you that the vaccumm is low and have you backed this up via a totaly seperate device?
With both factory and aftermarket ecu the problem is there, although the aftermarket masks it better.For the manifold vaccumm to decrease (pressure gets closer to atmospheric) either 1) the thottle is opening, or 2) the engine volumetric efficiency is falling. I would expect to see a higher MAP with a cold engine as the engines frictions in higher some more IMEP is required.
I have once had a crank sensor fault (was actually a damaged sensor ring tooth), that meant the ignition was ocasionally retarding (although of course the ems "showed" the incorrect value on screen) to such a degree that the engines BMEP fell so much that the throttle system had to open to eWOT (effective WOT, which at low rpm is very small angle) to maintain engine speed.
so some questions to think about:
Can you see what the throttle and ignition angle are doing during these events? (can you get a timing lamp onto the engine to see "True" ignition angle
how does the engine sound during such an event, does it sound like it is misfiring?
what sensor is telling you that the vaccumm is low and have you backed this up via a totaly seperate device?
I dont recall actually confirming timing is as commanded, but have no reason to actually warrant checking this. ( even playing with timing on the aftermarket ecu doesnt fix anything, if you get what I mean )
I can see no situation where ignition timing will be wrong. Although I guess the scenario you mention is possible.
OE uses crank/cam at all times.
Aftermarket syncs on startup, and thereafter only uses crank.
Manual throttle, so it is not moving. ISCV will perhaps compensate to an extent on the OE, with the aftermarket this isnt closed loop at all.
Both map sensor on engine, and mechanical gauge in car ( and Snap on Vac tester )
So in effect, confirmed on 3 independant gauges, and on different ports on the intake manifold.
Although just using in-car now, as it is correct.
Car has had enough probes attached to it !!
And yes, the oddest thing is, it doesnt sound like it is missfiring when the problem is there.
RPM can drop a bit, but generally the engine still sounds good.
When its running properly, it is one of the sweetest running Subarus Ive worked at ! and quietest.
Ripped highflow cat out of it the other day, even though it looked fine, again with no sound reason for doing so.
Even still all emissions are excellent. CO well below 1%, HCC circa 120 at idle, CO2 and O2 etc all perfectly normal ( cant rem exact numbers )
Fast idle, it would sail through MOT no problem at all.
During the problem, HCC does rise though, and I think O2 does too. So air is passing through the engine somehow.
I keep meaning to get him to scope the inputs to the coils, in case something is dropping out. Sure that hasnt been done yet.
oakdale said:
Poor manifold vacuum isn't just a mechanical problem.
The vacuum is caused by the resistance of the throttle plate/butterfly to the piston induction stroke and any problem that slows down or causes incomplete combustion on the previous power stroke will reduce the vacuum (varying ignition the timing has a large effect on vacuum).
What's the full history on this and are there/have there been any fault codes?
History far too long to document. I think original codes were missfire codes, but its been that long cant remember.The vacuum is caused by the resistance of the throttle plate/butterfly to the piston induction stroke and any problem that slows down or causes incomplete combustion on the previous power stroke will reduce the vacuum (varying ignition the timing has a large effect on vacuum).
What's the full history on this and are there/have there been any fault codes?
No codes at present. Codes are often misleading anyway.
Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff