Rough Low RPM Idle, won't stay running in morning

Alright I have searched this forum, among others to start pinpointing things and at this point I am at a loss. So heres what I got and I would like any input you have. It's a 1996 4.0

Truck through codes that it was misfiring on all 6 cyclinders, I have done the standard tuneup procedures and it still has the problem, the problem is this...

In the mornings the truck will hesitate in starting and then unless you keep the gas pedal down it will die, if you drive the truck with constant throttle to maintain it running after about 25 minutes it will maintain idle, however at this point it will idle very rough and lower than expected. I would like to tell you the RPM's but it is a base model with no tach. At the exhaust pipe it has a very distinct sputter that confirms the misfiring that could be evident. I replaced the IAC with no apparent change in running condition, when I disconnect the TPS the truck will almost cut off and then run as if the TPS was still connected without much change, the same happens when you disconnect the MAP. Lastly I have had the truck running and disconnected the CPS and it cuts off immediately.

Any thoughts on what to do next would be greatly appreciated. I had a 99, and another 96 that had a similiar situation at startup, as it would start and then die immediately, however they corrected themselves so I never had a logical solution to the problem.

The best strategy for these types of problems is component testing; don't start throwing parts at it. That is an expensive, frustrating, and normally pretty inefficient way of shotgun troubleshooting a problem. The fact that you have codes that point to all cylinders means you have a component common to all cylinders on the fritz.

If it were mine, I would definitely start by testing the TPS with a meter. The fact that you have to keep the gas pedal down kind of sounds like the problem may be related to the TPS. Get a manual for the procedure.

The throttle position sensor is connected to the throttle shaft on the throttle body. It sends throttle valve angle information to the PCM. The PCM uses this information to determine how much fuel the engine needs. The TPS is really just a simple potentiometer with one end connected to 5 volts from the PCM and the other to ground. A third wire is connected to the PCM. As you move the accelerator pedal with your foot, the output of the TPS changes. At a closed throttle position, the output of the TPS is low, about a half a volt. As the throttle valve opens, the output increases so that, at wide open throttle, the output voltage should be above 3.9 volts.

Your problem is not the CPS as you've found out. Other things to think about if you rule out the TPS would be testing the coil, all wires (especially coil in this case) and coolant temp sensor.

Good luck and post back what you find.

I have a 94 4.0 and it had a similar problem. It would idle real rough (sometimes to the point of dying) then it would choke it self and rev up. As long as I was driving (ie: not sitting at idle) it would run fine.

I changed my O2 sensor and it fixed the problem.

Try unplugging the O2 sensor and see if it makes a difference.

x2 on what everyone has said

only thing i have to add is, what is the battery voltage at idle when this is happening? a low voltage problem at idle can also cause problems.

I just cured a similar problemin my 98. Was the downpipe 02 sensor.

Okay I will add this, last night it was running I disconnected the down pipe O2 sensor and noticed no change in idle or while increasing throttle response. also this problem with starting is also a problem in the evenings at times as well, so to clarify it is not always a cold start or morning problem.

southerncruiser62 said:

Okay I will add this, last night it was running I disconnected the down pipe O2 sensor and noticed no change in idle or while increasing throttle response. also this problem with starting is also a problem in the evenings at times as well, so to clarify it is not always a cold start or morning problem.

Click to expand...

the engine needs to be at operating temps for you to unplug it and get a difference.

so if you unplugged it while at operating temps and there was no change, wouldnt that indicate that the sensor is not operating properly? i was under the impression that when you disconnect a good operating sensor that you should notice a change, if not than the sensor is not working properly

I am also getting incredibly bad gas mileage as well. Truck was at operating temperatures when O2 sensor was disconnected with no change.

Try connecting a vacuum gauge.
Read this site it may help you narrow down your problems.
You could have a vacuum leak.

http://autospeed.com/cms/A_2393/article.html

Update:

Okay so without an analog meter (yeah I read they are cheap, but at 110K, I figured it couldn't hurt to replace these parts). I just went ahead and replaced the following:

TPS Sensor
Downpipe O2 Sensor
IAC Sensor

No change at this point.

Any additional ideas, I have a rough low idle, and at the exhaust pipe it is clear it is perhaps missing consistently. Again, after just initially accelerating there is no evidence of a mis and at highway speeds everything seems to be operating efficiently.

I have got to figure this out.

Definitely test your coil; takes only the ability to read a meter and 10 minutes of your time! If coil checks out, take some resistance numbers on your coolant temperature sensor and compare your numbers to the CTS/Temp chart. Never hurts to verify fuel pressure numbers at the fuel rail.