Semi random high idle and running rich

Ok so I have an 88 5speed with 281k. Usually when starting the idle jumps to 2.5k and then creeps up to 3k and holds there. I've tried letting it run and seeing if it goes back or tapping the throttle, but the only thing that brings it down is turning it off for a few seconds and restarting it. Some times it takes a few tries. But after driving for a few minutes it will idle anywhere between 1.3 and 2.5, usually when comming to a stop or going in neutral. I have replaced iac, tps, and cps, I have checked for vacuum leaks. I recently warmed it up to running temp and got it to idle normal, about 850, and disconnected the iac. It still idles a little high, about 1k, after driving. Also I am getting very poor mileage and its running very rich. (Fuel pressure is at 31psi) I have also gone through the grounds and harness connectors on firewall and leaned them. Also I've dialed in the tps almost perfect, I think it's off by .001. I feel like I'm running out of options. I'm wondering if this engine is just due for a rebuild. Anyone have any ideas on what else to check?

Lots of ideas, have you run intake manifold and throttle body gasket vacuum leak tests.

Has the O2 sensor and wiring, power inputs (5 volts and 14 volts and ground) been tested,

My best guess so far (as I had this exact problem for months once that was hard to debug) is a very old dirty, maybe defective IAT Intake manifold Air Temperature sensor that is too dirty to respond timely or that is bad. Try cleaning it gently but beware the part is no longer made. It can be replaced with some wiring/connector work. But they have been revived with good solvent cleaning.

Also check the MAP sensor to throttle body vacuum hose closely for a leak, and check the fuel pressure regulator vacuum line for the smell of gas which would mean the FPR seal is bad and leaking excess gas into the intake.

I may have missed a few, but I think you got the others already.

I assure you it is not the engine!

so you were on to something with that IAT sensor.
i cleaned it and its acting much better for now. i took it for a little run and went around town, on the freeway, and did a short trail and it was idling normal 60% of the time the other 40% it was sitting at about 1100.
the o2 sensor was reading ok. i have a small exhaust leak at the #1 but the intake is ok. no vacuum leaks that i can find and the map is good too. fuel pressure regulator might be bad, there is a faint fuel smell there, so i will look into getting a new one.
only reason i thought it might be the engine is that it has some pretty good crank pressure and blow by. also it developed a lifter tick recently (surprised it made it to 281k before it started ticking). but i believe the rings are starting to go bad.
she is definatly running much better though, and a little more resposive too.

Never mind... Let it cool down for a few hours, then left for my girlfriends house and as soon as I started it it jumped right up to 2500 and sat there. Restarted it and did it again. Let it sit for about 20 second started it again, it went to 2500 again but after about 5 seconds it started to drop and idle normally. I'm going to get another iac in a few days. And see if that changes anything. If not I'm gonna save and replace ALL of the sensors. Found out today the knock sensor was broken as well. Just expect every problem right.

Check the relay that keeps the power to the computer for a few seconds when you turn the key off. That allows enough time for the computer to reset the Idle Air Controller back to the start position. If that does not happen, the rpms will jump up when you first start, then after a while the computer will realize the high idle and adjust the IAC.

For the life of me, I can't remember the name of the relay. It is 6am and I haven't had my 40 oz of coffee yet.

old_man said:

Check the relay that keeps the power to the computer for a few seconds when you turn the key off. That allows enough time for the computer to reset the Idle Air Controller back to the start position. If that does not happen, the rpms will jump up when you first start, then after a while the computer will realize the high idle and adjust the IAC.

For the life of me, I can't remember the name of the relay. It is 6am and I haven't had my 40 oz of coffee yet.

Click to expand...

I second that motion and add cleaning the contacts for that relay.

But I am curious, does the IAC relay set up at a setting when hot that if left there, causes a high idle when cold? Too far open? My best guess is yes as the air is thinner when hot in the intake manifold and takes a more open setting for the IAC when hot. Thus if the IAC is adjusting from cold to hot and then not adjusting for a cold start with engine shut off, that would suggest the relay or wiring not the IAC in my book.

cleaned my b-latch relay (and the other 3 around it just because). pulled the throttle body today and cleaned that along with the IAC. checked all the sensors again the only bad one is the knock sensor (cant really check it that i know of but it is broken) put a very thin layer of sealant on the TB on manifold and intake sides. did notice that there is a stripped bolt hole for the intake boot to the TB, the bolt on the IAC side on top that holds the intake boot to the TB. also cheked all the grounds again. they all look clean and good. also replaced the vacuum line to the MAP. checked the egr as well and its working properly too. it does look like the intake manifold could use a very good cleaning. despite all that it is still running the same. high idle at start to about 2500 and sits there either until it feels like dropping down or i restart it. it doesnt always high idle on start though. sometimes it idles perfect for awhile. but if i stall or it shuts off without turning the ignition off the next time you start it, it will high idle.

Have you run a complete ohm test of the IAT sensor hot and cold, in ice water and in boiling water and compared to the Factory spec? Done a wiggle test on it and the harness wires to see if the reading changes with motion, and checked the ground and input voltage to it while wiggling the wires?

The Renix jeeps wiring harness is notorious for loose, sloppy noisy ground wires that are not soldered or well crimped connectors that are buried in the harness. The Cruiser54 files and FAQs document how to find and fix those.

"so you were on to something with that IAT sensor.
i cleaned it and its acting much better for now."

So I believe i have narrowed it down to, of course, the TPS. Went to the bruiser54 list and just started doing everything. after spending hours cleaning and testing voltage with no avail, i started it and it started the idle went strait to 2500 so i just started pulling sensors only one that brought it down without killing it was the tps. strange thing is when i test the resistance and voltage it is perfect. im wondering is there a difference between the auto tps and the manual tps. oreilly shows 3 different tps, 2 auto trany ones and a manual trany. i have a manual trany but i believe the sensor i have is for an auto. it has both the 4 pin and the 3 pin plugs. the manual tps oreilly shows only has the 3 pin and the plug is directly attached to the tps not extended by wires. is it possible that i have the incorrect tps and it is throwing my readings off? i would love to just go pick up the tps but unfortunatly it is $123 and a little out budget at the moment.

No need for the manual TPS, waste of money. Every one uses the auto TPS to save money with no problem, but many use the dealer only part.

Disconnecting the TPS and getting the idle to drop is 100% normal, even with a good TPS and a high idle problem, or a bad TPS causing the high idle. You have proved nothing. Could still be a bad wire in the harness or loose connector from the TPS to the ECU/PCM computer.

Only way to prove the TPS is bad is to read the return voltage from the TPS to the ECU while the idle is high at 2500 RPM. If it is factory spec or close, it IS NOT THE TPS!!!!!

If the TPS is way above spec while the idle is at 2500 rpm, like say 1.0 to 1.5 Volts, either the TPS is bad, or there is bad wiring from the TPS to the ECU/PCM computer adding resistance to the return signal wire. You then wiggle the hell of the wiring harness under the dash and under the hood to see if the voltage and idle changes. If nothing changes, then it is likely the TPS is bad.

One last change you can do is take an ohm meter to the parts store, test the old TPS on the car first, disconnected when it was idling high, to see what the ohm reading is. The at the parts store test a brand new. If the ohms are close to each other, +/- 3% I would guess, it is not the TPS, if the ohm reading is off 20% or more then bingo it is the TPS that is bad.

Tip 5 passed the test?

Following

My 88 does something very similar.

Byron1988 said:

Following

My 88 does something very similar.

Click to expand...

Have you completed Tips 1 through 5 at www.cruiser54.com?

If not, you need to do so.

Sorry for the late response, started a new job and I'm working 60 hour weeks. Haven't had time to do much.
I did steps 1 thru 5. Every thing is clean and the sensors all seem to be in the correct ranges.
I have been driving my jeep to work every day and the problem seems to be getting worse. Defiantly won't idle below 1200 after driving it but sometimes it will idle at 750 only when I start it. But it usually sits at about 1500. Every morning I can count on it jumping up to 2200 - 2500 until I restart it a few times. I noticed the longer I let it high idle before restarting it, the more likely it is to idle correct when I restart it. Then as soon as I start driving its back to idling at 12 - 15. Also it smells like it's running even more rich. I'm wondering if it's a fuel pressure regulator or injector issue, although I get 31 psi at the fuel rail. But I only checked it running. I need to go rent the tester again and check how long the pressure holds after I shut it off. Also the vacuum line to the regulator, and the oil smell like fuel.
Trying to save some cash from this new job so I can throw some parts at the jeep but I'm still catching up on bills so it might be a week or two before then. I can afford one part, I just need to figure out which is my best bet.
Oh my knock sensor is destroyed too. Forgot about that. Didn't think it was super important. I always run 91 in it so it shouldn't be knocking or pinging. At least I don't hear it.

Don't throw parts at it. Knock sensor is not the issue.

How's this?

http://i1354.photobucket.com/albums/q690/cruiser54/Jeep Tech Photos/TB to MAP tube 2_zpsf5qva2l8.jpg

and this:

http://cruiser54.com/?p=280

Map is good too, re did the line and made sure it had a nice seal to both the map and the tb

Tip 5 has been done?

Bingo!!!

"Also the vacuum line to the regulator, and the oil smell like fuel."

The slightest smell of gasoline in the vacuum line at the fuel pressure regulator means the diaphragm is leaking fuel into the vacuum line and that will cause a high erratic idle etc!!!

Ecomike said:

Bingo!!!

"Also the vacuum line to the regulator, and the oil smell like fuel."

The slightest smell of gasoline in the vacuum line at the fuel pressure regulator means the diaphragm is leaking fuel into the vacuum line and that will cause a high erratic idle etc!!!

Click to expand...

Yes^^