I was wondering if anyone might have an idea as to why my long term fuel trim in high (+25% usually). This causes a minor nuisance at startup but otherwise I have no perfrormance or drivability issues to speak of. My fuel economy has never been worse though.
Here's the data...
RPM 751 (verified ok) Ign Timing Advance -8.5d Intake Air Temp- 12C TPS - 1% O2 bank 1 - 0.040 (switches to .8-.9 with no issues) O2 bank 2- 0.275 (seems slow to switch - maybe this is the issue?) Load value - 3.1% Long Term Fuel Trim- 25% MAP Sensor- 39 kPa Coolant Temp- 93C
in article snipped-for-privacy@no-mx.nodomain.com, J-Mech at snipped-for-privacy@no-mx.nodomain.com wrote on 2/24/07 11:31 PM:
Ok..
Here's the data... Is this better??
RPM 751 (verified ok) Ign Timing Advance: -8.5d Intake Air Temp: 53.6d F TPS: 1% O2 bank 1: 0.040VDC (switches to .8-.9 with no issues) O2 bank 2: 0.275VDC (seems slow to switch - maybe this is the issue?) Load value: 3.1% Long Term Fuel Trim: +25% MAP Sensor: 11.51 inHg (.39BAR) Coolant Temp: 199.4d F
It might be. Without a BARO reading, it is impossible to tell.
What is the short term fuel trim?
What are the long and short term fuel trims for the other bank?
What is the Mass Air Flow reading?
What happens to the readings during cruise?
Taken at face value, the +25% fuel trim indicates a lean condition. This can be caused by vacuum leaks, plugged injector poppets, low fuel pressure, wrong PCV valve, skewed MAF sensor, skewed O2 sensors, air leaks in the exhaust system ahead of the O2 sensors...
Andrew-does your scantool give you any fuel trim counts, ex.- 128 counts-above(higher or lower)? jerry @peru-motors.com ps also wonder if your EGR valve could be slightly/ partially open-not going completely closed due to carbon build up over time thus causing your hard start. Just a guess
Thanks J-Mech. Do you think a vac leak would lead to a 25% adjustment in long term fuel trim? I just finished checking every hose and gasket (water tested also) and I can't find a leak. I did have a partial crack on the vac reservoir though but I don't think it was serious enough to cause such a wide swing. I will rescan and get back if there's any change. I don't have any hard start issues any longer but the fuel trim starts at about +7 and tops out at
25 over after maybe 2 minutes of idling. A suggestion had been made that a top end motovac treatment might be the ticket? Any thoughts on this?
Thanks again! Andrew.
in article snipped-for-privacy@no-mx.nodomain.com, J-Mech at snipped-for-privacy@no-mx.nodomain.com wrote on 2/25/07 8:00 PM:
I think Jerry may have meant to say 'pulse width counts' (I could be wrong maybe he missed it altogether). Jerry had emailed me about it earlier in an effort to help out - he has been very valuable in the past with his input and experience. For the record, I can't get pulse width information from my scanner but I can get in from my scope. I haven't tested for pw on this vehicle before, maybe someone can help with which wires I should tap and what baseline I am looking for, etc? I'm not sure that checking the pw will help fix the issue though as I would expect an increase fuel trim to show up as increased pw (I could be wrong though).
Thanks! Andrew.
in article snipped-for-privacy@newsclstr02.news.prodigy.com, aarcuda69062 at snipped-for-privacy@sbcglobal.net wrote on 2/25/07 5:56 PM:
Thanks for the response. I really appreciate it. I will rescan it and let you know. As for the MAF reading, I didn't think this vehicle had a MAF sensor. If you can take the measurement some other way, I apologize. When you say you need a BARO reading are you speaking of the MAP sensor's output with key in engine off vs idling? As I stated earlier, I don't think there are any vac leaks but I may have missed something somewhere, the injector poppets are a definite possibility, PCV valves are fine (test ok, rattle, no leaks), MAF sensor out of whack is also a definite possibility (I need todo some impedence and voltage tests I guess), exhaust system is air tight as it was just redone (problem existed before as well but again, I will double check).
When you asked what the readings would be at cruise do you mean with the transmission engaged or in neutral (stupid question - sorry). I was going to take readings in every gear if that's at all helpful (just take freezeframes on-the-fly).
Once again, I appreciate all the help and support that you fine folks have been giving me. Andrew.
in article snipped-for-privacy@newsclstr02.news.prodigy.com, aarcuda69062 at snipped-for-privacy@sbcglobal.net wrote on 2/25/07 11:24 AM:
Shame on me for not using my computer glasses when I first read your post.
Yes, if not shown separately, BARO is the MAP reading key on engine not running.
Genuine GM part (PCV), not aftermarket?
Ignore what I said about MAF, I misread and thought you had a 96.
Not a stupid question. Cruise as in cruising on the road.
3rd or 4th gear at 1000 RPM, 1500 RPM and 2500 RPM keeps it simple. A vacuum leak does not show up in fuel trims as apparent at cruise speeds so the trim numbers are usually closer to center.
Thanks again. Here's some atmosph BARO data for you. I live at approx
350ft above sea level which translates to approx 29.16 inHg. I took a reading of 30inHg which translates to approx 10ft above sea level. Now bear in mind that I may have my current altitude wrong as I at the base of a
900-1000ft escarpment. Even so, if the MAP was out by 300ft above sea level equivalent, that wouldn't make a huge difference would it? I would think we'd be looking for 1000s of feet out to make a wide adjustment?? I'm actually amazed as to how accurate that sensor really is...
I will go back to the drawing board and do more tests. Thanks again! Andrew.
in article snipped-for-privacy@newsclstr02.news.prodigy.com, aarcuda69062 at snipped-for-privacy@sbcglobal.net wrote on 2/26/07 8:10 PM:
Idle test at 900 rpm (a little high b/c of my test runs I think) IGN back and forth from -1 to -14
25% LTFT TPS 0%
0 STFT LOAD 3%
10-11 inHg
**TPS ratings are low b/c of icy conditions - I 'crept' up to speed I didn't list O2 sensor data because there's no issue with high/low voltage or switching freq
Does this datastream seem pretty much to baseline/normal specs for what you would expect to see?
Thanks! Andrew.
in article snipped-for-privacy@newsclstr02.news.prodigy.com, aarcuda69062 at snipped-for-privacy@sbcglobal.net wrote on 2/26/07 8:10 PM:
You show a 35% fuel correction on the 1200 RPM run, that is way lean. The MAP readings, can't tell if thats a simple closed throttle reading or if they are during decel. If decel, they should read higher vacuum (lower MAP). Is fuel pressure in spec? That engine is under fueling by a bunch. You need to see if the O2 goes high and stays high at WOT high load... If I had this in my shop, I'd plumb some propane into a vacuum port, drive it and see if the propane brings the trims back in line and if there is a power increase. If so, I'd be on the phone to Linder for a new spider.
Fuel pressure is fine - 61psi with the key on engine off and dropping to maybe 55 after 30min. I don't think that a leak would take that long to drop the psi but I could be wrong. I've never checked the fuel pressure while driving but I suppose I could rig something up to make that work without killing anyone around me...
I don't know if that 30% correction was under decel but I would hazard to guess - no. I tried to lock in all the frame data while under light load.
I lied - I just rechecked and with the TPS flat, I was coasting and decel. For giggles, that run showed the following O2 sensor data:
Bank1 0.050V Bank2 0.300V
On another run with the tps at 5% (1449rpm) the O2 readings were:
28% adjustment in fuel trim while accelerating
Bank1 0.855 Bank2 0.330
You mentioned the dreaded spider replacement as I knew it might be a possibility. Would it be worthwhile to get a top end motovac treatment done after exhasuiting all other options?
Thanks again for your help and patience, Andrew.
in article snipped-for-privacy@newsclstr03.news.prodigy.net, aarcuda69062 at snipped-for-privacy@sbcglobal.net wrote on 2/27/07 8:38 AM:
If anyone is interested or still following, the latest on the beast is that last night VERY late, I did a Seafoam top end treatment (sucked about half the can up through the vac line from the booster). I let it sit in the top end of the motor for about 10 minutes and then I idled the motor for about
20min. The smoke was absolutely unbelievable! I thought someone was going to call the fire dept! Anyway, I limped it out to a country road and gave it a little punishment to clean out all the excess carbon. I brought it back to my 'shop' and scanned it again. The long term fuel trim is steady at about 7-8% (rpm between 680 and 720) and the short term % is nearly flat. I anticipate that this is due to a mild vac leak now or maybe a slightly lazy O2 sensor (it seems to stick low a little so I suspect it may be causing a slight false lean condition still). I am glad to report that the problem is no longer 'pegging' the ECM at 25%+.
I anyone is interested in 'filing' away situations for future reference... This may be a worthy memory. If you suspect a poorly running fuel delivery system, try this seafoam stuff (perhaps before costly injector/spider assy replacement) if you've never used it before. The responsiveness and idle of the motor is much more precise also after treatment and I assume my fuel economy will benefit also. The manuf suggests following up with 1/3 of the can in both the gas tank and the crankcase but I only treated the top end of the engine as it was my problem I suspected. For $11 a can, I have no complaints! I am going to go with the fact that there must have been a restriction in one of the poppets or perhaps an EGR issue (either way, the treatment solved the problem almost entirely).
I am going to replumb all the old dried vac hoses, new PCV gasket/elbow and valve from the stealership etc. - as well as pull and clean (or replace) my O2 sensors and see if I can get the fuel trim flat. This has been a very educational experience of which I have learned a great deal more about the individual sensor's authority affecting fuel trim and my diagnostic progressions. Once again, I appreciate all your time with this (everyone involved - especially Jerry and aarcuda69062). I will get in touch if I'm successful in completely flattening out my trim numbers but for the time being, I am confident we nailed the issue.
Take the best of care, Andrew.
in article snipped-for-privacy@newsclstr03.news.prodigy.net, aarcuda69062 at snipped-for-privacy@sbcglobal.net wrote on 2/27/07 4:24 PM:
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