TJ 4.0 hestitates and surges - follow up x2 to Independent Mechanics

Hey Reboot, there maybe several reason why any engine hesitates, surges etc.

1) Fuel - weak fuel pump, clogged fuel filter, poor fuel, fuel contaminants (water usually), stuck choke on carbureted engines, dirty injectors. Start with the easy fixes first. Fill up at a Brand name gas station with the recommended Octane gas. Dump a good quality fuel injection/carb cleaner into the tank. (Amsoil and Redline make an excellent product). Change fuel filter. Squirt some throttle body/carb cleaner into the intake. Start it up and squirt some more in working the throttle back and forth. If you have a pressure gauge you can measure the fuel pump pressure. If not the boys at the garage can check that for you. Even if it is in "spec" if it is on the low side, it may need replacement ( I had a Chevy Blazer that had that problem). Also inspect the fuel lines for kinks and fuel tank for any dents. ( I had a truck with a dented fuel tank and it could only pick up fuel when it was above the half full mark). 2) Spark - worn sparkplugs, coil, distributer cap, rotor, wires. Replace plugs ( I like NGK Iridium's myself), cap and rotor. Looks like you already had the wires replaced. If you have a lot of miles on the coil, replace that too. I usually replace any stock coil with one from Accell or MSD. Gives it a smoother idle and better throttle response when you go to a higher performance coil. 3) Air - clogged air filter, dirty throttle body. Replace air filter. Preferably with one from K and N. The dirty throttle body was taken care of when you squirted the throttle body cleaner into the intake. 4) Exhaust - Clogged or defective ERG (Exhaust gas recirculation), kinked or bent exhaust pipe, clogged catalytic converter. weak or worn O2 sensors. Inspect exhaust for dents. Replace the O2 sensors (most have 2 - one in front of the Cat and one behind) if the above steps haven't fixed your problem. Replace any kinked exhaust pipes. Remove and clean out your ERG valve. Your may have to replace it if it is badly worn. I am betting that the ERG is stuck open and that is why you are getting the majority of your problems when it is cold. Start with that.

Hope this helps... snipped-for-privacy@gojeepgo.ca

I would first suspect the connection on the TPS needs a clean just because it is a really common failure point that is heat sensitive and controls the shift points as well as the idle speed.

A good spray electronic contact cleaner is best, but even WD40 (Water Displacement spray) will work, just not last as long.

Mike

86/00 CJ7 Laredo, 33x9.5 BFG Muds, 'glass nose to tail > '99 TJ 4.0 Auto - within about 3 minutes of a cold start under part

When I first got my 88 MJ, I fought a similar problem for several months before I finally got most of it cured. Biggest improvement was a really thorough cleaning of the throttle body - don't forget to use a drill or pipe cleaner to get the port for the map sensor cleaned. Next was a complete tuneup - but that helped starting and high end more than anything. Cleaned all the electrical connectors on the throttle body, O2 sensor, map senor, and CPS. All helped some but light throttle just off idle was still problematic. A new O2 sensor fixed that (old one seemed to test good, but idle mixture was obviously rich). This thing had been rode hard and put away wet before I got it so everything was cruddy.

After all that, I still have one problem that I haven't figured out. Cold start, air temp in the 35 - 40F range, the damned thing is going to stall and die at the stop sign at the bottom of the hill. Just once, just at the first stop, just at that "in-between" temperature.

OK, I have the older Renix computer so that could be a difference but the point of this is that it was apparently a combination of little thing with each one contributing to the problem.

You seem to be saying that it happens during warmup and goes away when full temp is reached. Have you checked the temp sensor? The computer uses a separate sensor rather than the one that drives the gauge, so I would at least look to make sure that the computer wasn't getting a false temp and coming out of the open loop mode too soon.

well folks, i've tried many of the suggestions made w/o any success.

i had all of the TSB / IAC / MAP / CPS and IAT electrical connectors apart, cleaned both sides with QD Electronic Spray, blew them out w/ compressed air a re-assembled w/ dielectric silicone grease.

i had the throttle body off and cleaned it and took apart the IAC on the side and cleaned it., staining but no blockage.

i checked the spark plug wires after dark with a spray bottle of water and it looked good.

the driving experience when it hesitates and surges is like it momentary runs below the lean limit for combustion. when it hesitates, if I go to WOT it catches and pulls hard. no more hesitation happens at WOT. but if i take it back to part throttle the condition re-occurs.

from this I reason that the fuel rail pressure and injectors are OK - if not then the problem would persist at WOT.

i also deduce that it isn't a spark problem because the load on the ignition system is greater at WOT than part throttle and it runs fine at WOT.

if there is any problem with the TPS/MAP/IAT/IAC/CPS sensors would they throw a code? in other words - can i have one of these sensors bad and not find a code? (should i start back-probing them and comparing the values to the FSM?)

damn - the wife is starting to doubt my wisdom in buying this jeep because it was 'simple and easy to maintain and repair' (my previous words which I'm beginning to doubt).

i;m reluctant to start swapping parts because of the expense and i don't have confidence taking it back to the independent mechanic, they couldn't find it before.

reboot

THis might sound crazy, but i had a similar problem with my CJ. I has a 4.0 head and MPFI. on cool mornings it would do the saem thing you describe until warmed up. By luck I had to remove the exhaust manifold to take care on another issue and when I put it back together everything ran fine. A few months later it started the cold stumble again. I remembered the manifold incident, so I removed it once more checked it for flatness across the flange and found it to be a bit out. Not much but enought to not get a perfect seal against the cylinder head. I had the exhaust manifold surfaced, and put a new exhaust gasket on... my troubles disappeared. I figure, the slight leak, whan cold allowed fresh air into the exhaust that threw off the O2 sensor reading making it rev then stumble... Worth checking into