Running slightly rich / LTFT question
Thread Starter
Newbie
Joined: Jul 2016
Posts: 28
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From: Eastern PA
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?
Junior Member
Joined: Mar 2011
Posts: 79
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From: central pa mifflinburg 17844
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.
Senior Member
Joined: May 2017
Posts: 645
Likes: 34
Model: Cherokee
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.
Thread Starter
Newbie
Joined: Jul 2016
Posts: 28
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From: Eastern PA
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.
Senior Member
Joined: May 2017
Posts: 645
Likes: 34
Model: Cherokee
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.
Thread Starter
Newbie
Joined: Jul 2016
Posts: 28
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From: Eastern PA
Year: 1999
Model: Cherokee
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 .
CF Veteran
Joined: Nov 2017
Posts: 3,700
Likes: 238
From: Groton, MA
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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From: Eastern PA
Year: 1999
Model: Cherokee
Engine: 4.0
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