Running slightly rich / LTFT question
#1
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Year: 1999
Model: Cherokee
Engine: 4.0
Running slightly rich / LTFT question
'99 4.0, 96k miles, 99% stock (swapped to 4-hole injectors). New O2s (NGK), new exhaust from manifold back, otherwise all original senors.
I think I am running a little rich, but not sure what (if anything) to do about it. Exhaust smells rich and my long term fuel trim confirms it. Here are my #'s at operating temp:
LTFT:
@ idle -10.9
@ 2500 rpm -7.8
STFT:
@ idle -3.1
@ 2500 rpm +3.2
Intake pressure:
@ idle 5.2 PSI
@ 2500 rpm 3.6 PSI
@ key "on", engine off 14.4 PSI
Vacuum:
@ idle -9.6 PSI
@ 2500 rpm -10.9 PSI
I was thinking to replace the MAP sensor because I've read a failing one could contribute to running rich. I see 14.4 atmospheric pressure, subtracting 9.6 vacuum, I should read 4.8 at idle but I have 5.2. Not a smoking gun (0.4 PSI off), but maybe enough to cause this? It's pretty spot on @ 2500 rpm (0.1 PSI off) but runs less rich there. Also, my oil smells heavily of fuel. Any advice?
I think I am running a little rich, but not sure what (if anything) to do about it. Exhaust smells rich and my long term fuel trim confirms it. Here are my #'s at operating temp:
LTFT:
@ idle -10.9
@ 2500 rpm -7.8
STFT:
@ idle -3.1
@ 2500 rpm +3.2
Intake pressure:
@ idle 5.2 PSI
@ 2500 rpm 3.6 PSI
@ key "on", engine off 14.4 PSI
Vacuum:
@ idle -9.6 PSI
@ 2500 rpm -10.9 PSI
I was thinking to replace the MAP sensor because I've read a failing one could contribute to running rich. I see 14.4 atmospheric pressure, subtracting 9.6 vacuum, I should read 4.8 at idle but I have 5.2. Not a smoking gun (0.4 PSI off), but maybe enough to cause this? It's pretty spot on @ 2500 rpm (0.1 PSI off) but runs less rich there. Also, my oil smells heavily of fuel. Any advice?
#2
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Year: 2001
Model: Cherokee
Engine: 4.0L HO
map sensor did same for mine, also another thing possibly your valve seat are going bad is there any oil blow by in your air filter from your valve cover.
#3
Senior Member
Did you check fuel pressure at the rail ? TPS readings ? Also , have you checked your compression recently to know the condition of the engine that the fuel/ignition system is trying to possibly comprensate ? A ton of variables here to look at. Assuming you’ve checked for any vacuum leaks or exhaust leaks even though it’s new it does happen a lot.
#4
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Year: 1999
Model: Cherokee
Engine: 4.0
No oil on the air filter. Did not check fuel pressure or TPS. But I don't have starting, running, or idle issues, just the double digit negative LTFT. Would a bad TPS contribute to that? Have not checked compression, but that's a good idea. I'll put that at the top of my list once the weather breaks. I did visually check for vacuum leaks, thought I found one on the line leading back to canister by the rear diff and repaired it, but noticed no change. I've been considering taking it in for a smoke test just to be sure from the vacuum angle. Same with exhaust, no obvious leaks that I see - at least pre-upstream O2 which is all that should matter right? Rest of the exhaust is not welded just clamped and may be leaking a little.
#5
Senior Member
No oil on the air filter. Did not check fuel pressure or TPS. But I don't have starting, running, or idle issues, just the double digit negative LTFT. Would a bad TPS contribute to that? Have not checked compression, but that's a good idea. I'll put that at the top of my list once the weather breaks. I did visually check for vacuum leaks, thought I found one on the line leading back to canister by the rear diff and repaired it, but noticed no change. I've been considering taking it in for a smoke test just to be sure from the vacuum angle. Same with exhaust, no obvious leaks that I see - at least pre-upstream O2 which is all that should matter right? Rest of the exhaust is not welded just clamped and may be leaking a little.
#6
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Year: 1999
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Engine: 4.0
much like a good recipe for dinner ..your engine all has to work together to create the perfect conditions for fuel trims etc. Tps is part of that equation . IF the tps is reading inaccurate to actual throttle position the engine compensates for more/less air than is actually there . A bad fuel pressure regulator same difference if the engine was receive too much or too little fuel the fuel trims would adapt to compensate . If your ignition system was weak and the cylinders weren’t being burned as efficiently as they once did yet again same scenario. Your trims aren’t terrible so I suspect this could be as easy as a little bit of maintenance . Cleaning the throttle body a possible tune up etc .
#7
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Year: 1995
Model: Cherokee(XJ)
Engine: I6 4.0L
How long ago did you do the injector upgrade?
It would be a shame if you had anything less than near perfect compression on an I6 with less than 100k on it.
It would be a shame if you had anything less than near perfect compression on an I6 with less than 100k on it.
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Year: 1999
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