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My grand Cherokee has low engine vacuum at idle. It's 17" inhgs. I've noticed the map sensors baro reading at koeo is either .68 to a full 1.0 from actual baro reading for the day. Could that be a cause of the low vacuum. I can't find any vacuum leaks with water test.
Hi Jason, have you got access to any scan DATA? A look at fuel trims and 02's at idle and 2.5 -3k revs would be good to see, might be able to identify if you have a vacuum leak this way.
It's not the lowest vacuum reading I've seen, is it a steady needle on your gauge?
Calling upon my years of experience, I froze at the controls. – Stirling Moss
Needle is steady. 02s keep switching during higher revs and while driving during high load applications. Long term trim at idle is at positive six and seven percent. During revs both banks go to 0 percent and while cruising during higher loads go negative numbers like five or six percent. Short term trims keep switching +2 to - 2 during idle and never go positive during heavy acceleration but go negative during a deceleration. Sounds like a fuel issue too me but my snapon fuel gauge kit needs a new gauge on it. Last time I used it the needle was stuck on 10 psi and I opened it up too try and fix it and really screwed it up.
A fuel pressure reading certainly wouldn't hurt, I'm wondering if it's a breathing problem, I mean like a partially blocked cat or even a particularly dirty air filter, I have myself over looked the simple things sometimes as I've ever got more involved in more advanced diagnostics
Calling upon my years of experience, I froze at the controls. – Stirling Moss
Thank-you very much Rick. Even with the 02s not going rich during acceleration and short terms not going positive sometimes going negative during acceleration, when you floor the jeep it will push you back in the seat.
Well thinking with no big drivability issues perhaps a leak down test just to confirm it's not worn rings or weak springs, poss pinch off the brake servo hose for a sneaky vacuum leak there, I mean they are real borderline bad readings on both your idle vacuum and fuel trims, although the the intermittent negative fuel trims during accel is abit of a curve ball, you said you had switching on the 02 so we know it's not lying. How's your mpg? How many miles on the engine?
Calling upon my years of experience, I froze at the controls. – Stirling Moss
134,000 miles. I've noticed I get less miles per tankful of gas. With the steady vacuum needle at idle being low, if you just barely touch the gas it goes up to 19inches of vacuum and at the same time long term trims drop too zero.
It sounds indicative of a vacuum leak, does the car have a long crank time? Thinking along the lines of egr valve letting by. Perhaps a smoke test if you have access to a machine, or waving abit of propane around whilst looking for changes to the mixture (Been wary of the obvious dangers using this method)
Calling upon my years of experience, I froze at the controls. – Stirling Moss
No long crank time the first time I start it in the morning but I noticed if I start it and pull it up a hundred feet or so and turn it off, when I restart it their is a very long crank time sometimes it takes three tries. It sounds more and more like a fuel issue to me now that I'm talking too you about it. Can a low fuel pressure(lean condition) cause low engine vacuum? No access to smoke machine
Hmm low fuel pressure, low Vacuum? I don't think I can see that happening, again try those tests, a good look at the obvious stuff like your throttle body, and maybe for peace of mind get that fuel pressure tester fixed and give that a go, need to get some direction really
Calling upon my years of experience, I froze at the controls. – Stirling Moss
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