Try...try...try...again

Stats: 90 Renix 4.0 with AW4 auto tranny.


Things done so far:Symptons: Intermittant very rough idle. Engine makes WUP WUP WUP sound and shakes Pushing down on throttle doesn't help. No power if you try to pull out The only way to get her to run right is 2-3 restarts, then everything Ok.

I'm out of ideas and hope somebody can help. I'm thinking IAC which I have cleaned and oiled but have no way to test. Any thoughts?

I'm having this same problem and will be looking into it tomorrow, I'll lt you know if i find anything.

I had a CPS that was right at the outside allowable ohm tolerance ( 293 ohms engine hot), it acted like that. Sometimes it would run OK, other times it wouldn't (it would sometimes barley run at all, much like a disconnected MAP sensor), it seemed to be heat related. At a certain engine temperature it would just act up. This was my second CPS, the first CPS that went bad, the motor just refused to start. The ohm test is 200 ohms plus or minus 75 ohms, most good ones test out close to 200 though. Test it a few times, after turning the motor (tone ring) over to a new spot.
When my IAC was sticking, the very poor idle (missing, dieing and bucking) would disappear if I depressed the throttle past idle. The IAC only seemed to affect much at closed throttle, idle, coming to a stop or starting.
I've had the CPS connector oxidize and cause resistance. I clean it periodically and leave it dry, with no di-electric grease or WD-40. Sometimes spraying it out with contact spray isn't enough, I had to find a very small brush and scrub on the connector pins. I usually notice it's dirty when my starts take longer (more revolutions). Though the CPS isn't the only cause of that problem.
There is a kit to shorten the CPS wire run through the harness. If the wiring between the CPS connector (or through the cPS connector) and the ECU has excess resistance, it will act just like a bad CPS.

Heat-related failure of the CPS or MAP can cause something like that (we can just about rule out the TPS, but it should be checked anyhow.)

The catchy thing about heat-related failures is that they're a pain to catch. Here's two methods I've used before with good results...

1) Get out your other half's hair dryer. Heat the suspect component (if you have an IR thermometer, stop at about 200*F. If you don't, drip small amounts of water on it at intervals and keep heating until it boils off instead of evapourates.) Start, and see if failure is reproduced.

2) Go to your electronics supply house, and pick up some "Freeze Spray" (electronic techs use it for just this purpose, just on a smaller level.) Get engine and engine bay up to operating temperature, and then spray suspect components and see if failure halts. I think freeze spray goes for about ten bucks a can, but it's worth it in saving your hairline!

That's how I shoot heat-related failures - either heat the component up on its own, or cool it off while it's running. Either way, you should be able to isolate the thing.

In between tests, make sure you allow time for all components to become roughly equal in temeperature again (either let the engine continue to idle and heat up the sensor, or let the sensor cool off before you heat up the next one.) This is critical - you only want to thermally stress one component at a time!

Canned air works the same as "freeze spray", just turn the can upside down. Might be a few bucks cheaper....

BTW, that's an awesome idea 5-90....I swear I learn something new from you every day.

8Mud said:

I had a CPS that was right at the outside allowable ohm tolerance ( 293 ohms engine hot), it acted like that. Sometimes it would run OK, other times it wouldn't (it would sometimes barley run at all, much like a disconnected MAP sensor), it seemed to be heat related. At a certain engine temperature it would just act up. This was my second CPS, the first CPS that went bad, the motor just refused to start. The ohm test is 200 ohms plus or minus 75 ohms, most good ones test out close to 200 though. Test it a few times, after turning the motor (tone ring) over to a new spot.
When my IAC was sticking, the very poor idle (missing, dieing and bucking) would disappear if I depressed the throttle past idle. The IAC only seemed to affect much at closed throttle, idle, coming to a stop or starting.
I've had the CPS connector oxidize and cause resistance. I clean it periodically and leave it dry, with no di-electric grease or WD-40. Sometimes spraying it out with contact spray isn't enough, I had to find a very small brush and scrub on the connector pins. I usually notice it's dirty when my starts take longer (more revolutions). Though the CPS isn't the only cause of that problem.
There is a kit to shorten the CPS wire run through the harness. If the wiring between the CPS connector (or through the cPS connector) and the ECU has excess resistance, it will act just like a bad CPS.

Click to expand...

She does seem to have a longish crank time when starting up. Just the other day when running errands she was cranking slower and slower and I thought UH OH, MY ALTERNATOR IS GOING BAD. On the last startup at the post office I barely got it started. Got home and put battery on charge--charger said full. It started right up, disconnected charger and the voltage was near 14 volts. Talk about Ghost in the Machine! BTW CPS resistance around 225 ohms and with meter on A/C I get .6 volts when I crank engine. I guess my starter is dying now.

I got a good schematic and pulled my ECU plug. Used a paper clip to jump each sensor pin (circuit loop) individually checked each sensor circuit for resistance. When I found excessive resistance in a loop, I traced the cause down. I found a bunch of resistance in various circuits.
I usually replace senors as a last resort, one because of the cost and two because they often aren't the problem at all.
My last big find was the ground for the TPS (ECU side) The ground wire was just sitting in a crimp (spice) that had never been crimped closed.
If the sensors check OK, my next step is grounds. Don't be affraid to shale the harness some. I avoid any sort of coating on the connector pins. Except for conductive paste on the ones I rarely open.

Don#t forget the obvious, like a CPS wire cooking on the exhaust or the fuel rail harness where it runs down the front of the motor cooking on the exhaust or rubbing through. The relay block corroded beyond bad and other common stuff.

Something often overlooked is to run tests while it is acting badly. For instance the O2 sensor could be causing your problem. I would grab the analog volt meter and run tests while it was running rough, testing the O2 and CPS voltage, even disconnect the O2 sensor while it is running rough to see if it stops running rough. Same goes for other sensors. Also check for a hot wire getting grounded during the upset condition, volatge drop anywhere.

IAC problem would not cause your symptoms.

Ecomike said:

Something often overlooked is to run tests while it is acting badly. For instance the O2 sensor could be causing your problem. I would grab the analog volt meter and run tests while it was running rough, testing the O2 and CPS voltage, even disconnect the O2 sensor while it is running rough to see if it stops running rough. Same goes for other sensors. Also check for a hot wire getting grounded during the upset condition, volatge drop anywhere.

IAC problem would not cause your symptoms.

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Didn't really think it was IAC, though thanks. Tomorrow AM have to replace starter, ordered one today NAPA. My ABS just went out, it comes and goes. Could be the relay overheating. Will open up bottom of dash and pull ECU tomorrow. I am not reading proper resistance at the diagnostic block under the hood for the ECU ground. Actually can't get the digital meter to give me a constant reading which could be a bad connection (oxidized) somewhere? Does anybody have a good pin out drawing of the ECU? The one I have doesn't show the ABS, or I can't make it out. (Poor quality down load of a scanned page I think.)

I don't look at this Jeep as a PITA, I look at it as a uh lifestyle decision.:tears:

I also had some CPS problems a while back. The original would run rough and die when hot. I used a long funnel to cool it down with water, add water, smoothed right out and kept running. Stopped water and it died.
So I got a new one and I had the same problems after a few days. Pulled the new one, checked resistance when cold, it was fine. Popped it in the oven @ 200 degrees, no resistance. I had a bad "new" CPS.
Got another one, tested it in the oven, third time was a charm.

F.W.I.W. how I got to the top bolt without too much effort; about 3 FEET of rachet extensions fed from behind the transfer case in this order (starting at the upper CPS bolt). 1) short wobble socket filled with grease to keep the bolt from falling. 2) 3 inch extension. 3) wobble end extension. 4) whatever else you have to reach past rear of the T.C.

Replaced dead starter. ABS pump completely dead. The dealership is nice enough to preorder one without seeing Jeep since they're 11/2 hours away. I have thought this problem might be voltage related since the ABS Pump kicks on when you startup and old starter could have been an above average current draw. I did notice it seemed to happen more often when A/C was on on startup (more current draw).
Added more grounds, added ground directly to ECU harness and to diagnostic block pin3, (the six pin block) reasson being I couldn't get a good reading at pin 3 the other day. This is ECU ground and same ground for Fuel Injectors! Also ohmed neg wires on injectors. They checked out.
If I figure this out I'll post the cure! A good drawing of ECU pin out would be really helpful.

My friend WUP WUP WUP is still with me.

Have you checked the EGR? What about the ignition switch? or a loose fuse?

Checked compression?

Ecomike said:

Have you checked the EGR? What about the ignition switch? or a loose fuse?

Checked compression?

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The EGR checked out fine. This isn't a heat related issue if I gave that impression my mistake. Found one bad fuse marked 'ABS" 2 amp probably the light.:spin1: How do I check out the Ignition Switch? What do I look for?? What would a bad ignition switch have to do with a rough idle?



I checked the ECU physically, no obviuos damage. I also re-ohmed the followingSo now its time to either open up the dash again and do a pin by pin test on the ECU or replace a sensor? Oh, can't check compression--no gauge, could I have a bad head gasket and losing compression? A restart wouldn't fix that symptom would it? Going to dealership on Friday to get new (and free:woohoo: ) ABS pump installed.

PuddinHead said:

The EGR checked out fine. This isn't a heat related issue if I gave that impression my mistake. Found one bad fuse marked 'ABS" 2 amp probably the light.:spin1: How do I check out the Ignition Switch? What do I look for?? What would a bad ignition switch have to do with a rough idle?



I checked the ECU physically, no obviuos damage. I also re-ohmed the following

MAT SENOR--NORMAL RANGE
CTS SENSOR --NORMAL RANGE
CPS--210 OHMS AND GENERATES .6 VOLT AC WHEN UNPLUGGED AND CRANKING ENGINE
MAP SENSOR--4.98 VOLTS SUPPLY, GROUND GOOD 1.8 VOLTS SIGNAL @ IDLE
CKS SENSOR 4.98 VOLTS SUPPLY, GOOD GROUND, VARYING SIGNAL ON AC AND DC SCALE WHEN ENGINE RUNNING.
TPS NEW AND WORKING OK
IAC SEEMS TO BE WORKING OK ALTHOUGH IDLE IS SOMETIMES LOW
O2 SENSOR READS 10.8 OHMS ON HEATER--IS THIS OK?????





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So now its time to either open up the dash again and do a pin by pin test on the ECU or replace a sensor? Oh, can't check compression--no gauge, could I have a bad head gasket and losing compression? A restart wouldn't fix that symptom would it? Going to dealership on Friday to get new (and free:woohoo: ) ABS pump installed.

Click to expand...

I was thinking maybe you have a valve seal going bad, and causing compression to come and go as the valve rotates, a long shot I know.

All your sensors sound OK, O2 sensor heater is OK. IF you suspect the O2 sensor or wiring, just disconnect it during the diagnostics. It will run open loop, rich but will not cause a rough idle or miss. A funky O2 sensor or O2 sensor wiring can cause a miss when connected.

New idea! Put a vacuum gauge on it, and see what the vacuum is doing during the wup wup wup. It might give us a clue.

Have you checked the Ballast Resistor to the Fuel pump? White ceramic thing, front fender drivers side, with 2 wires. If it is going bad it can starving the fuel pump for power at times until it dies completely.

PM me with your email address.

Oh, get a flash light and look under the dash top of the steering column for the wiring and ignition switch, wiggle those wires while it is running. Wiggle the key ignition switch while running to see if the problem comes and goes. If it does the ignition switch (not the key lock mechanism) is going bad, or the wiring to it, or both.

My Jeep had the same symptoms when one of the fuel injectors would bleed all the fuel out of the rail when the engine was shut off, and flood the engine. Does the engine clear out when you give a lot of throttle to clear out all the fuel.

Ecomike said:

I was thinking maybe you have a valve seal going bad, and causing compression to come and go as the valve rotates, a long shot I know.

All your sensors sound OK, O2 sensor heater is OK. IF you suspect the O2 sensor or wiring, just disconnect it during the diagnostics. It will run open loop, rich but will not cause a rough idle or miss. A funky O2 sensor or O2 sensor wiring can cause a miss when connected.

New idea! Put a vacuum gauge on it, and see what the vacuum is doing during the wup wup wup. It might give us a clue.

Have you checked the Ballast Resistor to the Fuel pump? White ceramic thing, front fender drivers side, with 2 wires. If it is going bad it can starving the fuel pump for power at times until it dies completely.

PM me with your email address.

Oh, get a flash light and look under the dash top of the steering column for the wiring and ignition switch, wiggle those wires while it is running. Wiggle the key ignition switch while running to see if the problem comes and goes. If it does the ignition switch (not the key lock mechanism) is going bad, or the wiring to it, or both.

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I unplugged the O2 sensor the last time the engine acted up. No change. If I open any of the vacuum lines during WUP WUP it seems to help, but only because idle speeds up. What vacuum reading should I get? I soldered the leads to the ballast resistor and it reads 1 ohm, but I'll double check, looks okay I guess I could bridge it out just to see. I'll check out the ignition after I get back from dealership tomorrow. One more thing. Ahem. The catalytic converter was rattling a few weeks ago and I unbolted the exhaust at the crossmember and extracted part of the guts of the Cat (ceramic wafer stuff with very small holes). Cat looks pretty new on the outside (had Jeep 1 yr.). No emissions here so thinking of pulling it off and either gutting it or replacing with pipe. This seems to point to O2, but O2 may have been replaced by PO. Thanks Ecomike. Funds a little tight here, so I am going slow.

IH392 said:

My Jeep had the same symptoms when one of the fuel injectors would bleed all the fuel out of the rail when the engine was shut off, and flood the engine. Does the engine clear out when you give a lot of throttle to clear out all the fuel.

Click to expand...

To be honest haven't tried that, but that IS a good idea. I replaced the injectors with new ones previously and maybe one of those injectors is leaking. I just can't remember if this problem started before or after the injectors, but I think after.:thumbup:

Ecomike said:

I was thinking maybe you have a valve seal going bad, and causing compression to come and go as the valve rotates, a long shot I know.

All your sensors sound OK, O2 sensor heater is OK. IF you suspect the O2 sensor or wiring, just disconnect it during the diagnostics. It will run open loop, rich but will not cause a rough idle or miss. A funky O2 sensor or O2 sensor wiring can cause a miss when connected.

New idea! Put a vacuum gauge on it, and see what the vacuum is doing during the wup wup wup. It might give us a clue.

Have you checked the Ballast Resistor to the Fuel pump? White ceramic thing, front fender drivers side, with 2 wires. If it is going bad it can starving the fuel pump for power at times until it dies completely.
PM me with your email address.

Oh, get a flash light and look under the dash top of the steering column for the wiring and ignition switch, wiggle those wires while it is running. Wiggle the key ignition switch while running to see if the problem comes and goes. If it does the ignition switch (not the key lock mechanism) is going bad, or the wiring to it, or both.

Click to expand...

Back from the dealership (3 hr round trip), New ABS pump works great and can barely hear it, hardly ever runs. New starter I put in makes for quicker starts. Mechanic check fuel rail presure for me @ dealership running about 35 psi. IH392's suggestion seems resonable but the engine is running great right now. We'll see how she starts in the morning. With 2 devices drawing extra current (old ABS pump and Starter) I wonder if the problem was with a confused ECU? Meaning undervoltage. It seemed to do it more often if I had the A/C turned on.

I'l check the ignition switch ASAP.:eyes:

You do know that a cat honeycomb can come loose and move around in such a way that it periodicaly blocks the exhaust flow, and then roll and work again right?

Some of you last posts have me thinking of trash in the IAC throttle body port and a sticking IAC.

Could you have a week vacuum hose colapsing under vacuum to the MAP sensor?