View Full Version : Unstable RPM's when on throttle, and idle

April 21st, 2015, 14:10
I just wanted to post to see what everyone thinks i should check. I'm leaning towards the TPS, or a fuel issue.

Here is a video of what it does when the throttle is very slowly applied until all of a sudden it takes off. It acts just like a carb that its idle circuits are clogged up making it lean out.


Not shown in the video is when it is at idle and starts acting funny.It will idle fine for a little while and then just start to have a slight miss. The idle control opens (very noticable wistle in intake as its opening to correct for whatever is going on) It only does it now and then. I haven't had the engine in the jeep for long so i can't say on the history of how it drove before. I have the front O2 sensor installed, but the rear isn't yet as i just finished up the exhaust today and thats when it started to act funny.

So far the plan is to test the TPS, install the rear O2 sensor, and if that isn't the problem i'll have to check the fuel pressure. Am i missing anything? I may also pull the brake booster line and plug it to make sure its not getting a vacuum leak from the booster.

April 22nd, 2015, 12:22
Just wanted to post an update. I checked the TPS today. It looked smooth the whole way through. So I pulled the Front O2 to see if it was causing the problem. did'nt help. SO i started to pull injector plugs as it was at idle and playing with the throttle a little to see if i could figure out how many cylinders had been causing it, or maybe find what ones. After i pulled 2-3 at different times and plugged them back in i tapped the fuel rail a few times with a hammer at different spots and the side of the injectors of the ones that i could reach. Anyway. after that it is acting fine. I started and shut it down and it was fine the whole time. It had a high idle problem just after i did the above with the injectors/fuel rail but that settled down after 2-3 restarts.

So either it was water in the fuel (sat for a long time) dirt in an injector/injector sticking? or the PCM was relearning since it has sat for 1.5 years with no power on the pcm.

Hopefully it stays straightened out.

Now all i need to do is go over the brakes, put on the lift, finish up the exhaust, and paint the front grille/bumper and rear bumper.

April 24th, 2015, 05:31
Ok. This morning i installed the battery into it again and fired it up. It ran ok for about 1-2 minutes while i went into the house and i could hear it surging trying to keep idling. It dips and then revs, and contiues that cycle for a little then it will settle into a very cammed up engine idle, i'm talking like super rough but not surge anymore. Anyway i tapped the fuel rail again, no change. So i pulled the #1 cylinders injector plug and plugged it back in and it smoothed out? Not sure what thats indicating, but something is going on. I need to wait for it to do it again and see if it matters what injector plug i pull.

April 24th, 2015, 12:44
i had a similar issue with my new-to-me renix last year. i found that wiggling the first three injector plugs or the harness would fix the problem for a bit.
i ended up unplugging all six injectors, using a pick to close up the contacts a little so they'd grab the injector pin better, cleaning all of them(injector and harness side) really good with deoxit contact cleaner, and then spraying each one with deoxit gold contact enhancer.
it's been just fine since then....been at least eight months or so now without any further issues.

April 24th, 2015, 13:57
Thanks for the reply. I will give that a shot. Its just odd that for a few minutes its fine, and then its all rough and unstable.

I bet the IAC opening is trying to make up for the missing cylinders by letting the good ones eat.
. I'm not sure that it could be fuel as the one small trip i took it on the road it ran fine.

April 24th, 2015, 14:27
I had a wacky idle and intermittent missing with various cylinders on my 96 three years ago. It happened after a mouse got under the hood and chewed through a couple of injector wires. Replaced the wiring, but not much better. Changed the computer with a wrecker unit and problem solved. The injector wires must have shorted something in the computer.

Ran fine for a year and a half and then I thought the problem returned.

This time, I checked the wiring and all was fine. Did a compression test and found low compression in#1 and #3. Pulled the head, valves were not closing fully with lots of pitting around the seats. Took the head off the old parts jeep, lapped the valves, new springs and seals, stuck it on, perfect ever since.

So - two totally different causes, same effect.

April 24th, 2015, 16:14
I'm leaning towards a PCM issue. I just took it for a beating on the road this eve for 3-4 miles. With the converter locked in 3rd if i slowly roll into the throttle just hard enough until reaching wide open to keep it from unlocking it doesn't sputter miss or anything, so its not a fuel volume issue.

And the PCM thing may very well be possible since a few weeks ago when i dropped the engine into the jeep i spent 3 days figuring out why it wouldn't prime the fuel pump. Ended up being all of the grounds had been cut off the firewall harness and i missed it when i dropped it in since it was a 6 month long project and i went all winter without touching it.

I have a PCM out of a 98 i may toss in it when i get time next week.

I'll finish putting the front clip together before i fiddle anymore with the idle issue.