ToothMechanic

Hi guys,

My new to me Jeep has long had a rough idle that I haven't been able to fix.

The check valve on my fuel pump recently failed and I was having long starts. Could alleviate this by the poor mans prime. I had a mechanic replace the fuel pump with a Bosch one.

I also had them replace the injectors. The Jeep ran like **** afterwards and I realized that I had been sold the wrong Bosch injectors. Was struggling to start, stalling, etc..

I put the old ones back on and now the Jeep is realllllly struggling to start when warm. I will have to crank it for a long time and give it some gas, then a super rough deep idle for 10 seconds or so.

Doesn't make sense to me because it did not have this issue previously and the only part that has been change (and remains changed) is the fuel pump.

I put a fuel pressure gauge on the rail. It will stay up at 45 for a while then slowly (maybe like 20-30 minutes get down to like 30). Before it was just plummeting right after I turned the car off.

On a cold start it starts ok.


Any thoughts?

Idunno

Idle air controller. Read up on the symptoms of a failing controller and see if they match yours.

ToothMechanic

Thanks for the reply.

I tested the IAC as much as I could with a multimeter. I also cleaned it. Everything seemed normal.

The rough idle does seem like a symptom of it and so does the stalling. Worth noting that the Jeep only stalled a couple times at really low speed. it hasn't done it again since I changed the injectors back to stock.

The main problem still remains an extremely hard start when warm/hot....and I'm not aware of the IAC having any relation to that?

Idunno

The IAC moves to predetermined positions based upon the moment's reporting by other sensors, including the upstream 02, temp and throttle, to the computer. If the IAC is not moving relative to the current condition of the engine you can experience difficulty starting. It creates an air bypass of the throttle body plate when the throttle (gas pedal) is fully closed (up). This permits the entrance of air needed for a proper starting fuel/air mix ratio. If it does not open this air bypass, for whatever physical internal failure prevents its operation, the proper starting fuel/air mixture cannot be produced. When you removed the IAC to inspect/clean it was it fully extended? This is the closed position, meaning that the air bypass of the throttle plate is closed off. When the car is cold and the key is turned to the on position the IAC should retract to a position allowing air to bypass the throttle plate. Every time you reduce the RPM to idle the IAC sets it. I recently did the same thing you did and even though the thing looked clean and assumed to be functional, the plunger was fully extended. I experienced long cranking and a stumbling idle. That was a cold engine. Replacing it restored my normal idle.

If the foregoing isn't exactly what you're experiencing then look at the symptoms of a failing upstream 02 sensor. A bad 02 can give you a stumbling hot idle. Read up on it's symptoms.

ToothMechanic

To be honest I wasn't sure how to evaluate whether it was fully extended or not. I'm not familiar with working on them.

I guess I will replace it just to see, it's not an expensive part or difficult job.

Is there anything else you would suspect base on what I reported?

ToothMechanic

Worked on the Jeep today.

First thing I did was go buy a new IAC valve. After putting this on I noticed no difference.

I took the fuel rail back off and checked to see if any of the injectors were leaking, which I thought could be causing my long starts. Nothing was leaking. I put everything back together and installed a heat shield that I had ordered (wrapped fuel rail/injectors and covered manifold). The Jeep's thermometer is reading higher now (just below 210), but I had ZERO problems with any of the hot starts all day today. So I can only assume my long crank was heat soak.

However back to the drawing board with my rough idle. The Jeep began throwing the cylinder 1 misfire code again today. I took the spark plugs out and every cylinder looked fine with the exception of cylinder #1, pictures here of what all the others looked like vs cyl 1: https://imgur.com/a/yROYYw5

Worth noting that I redid the ignition system (plugs, wires, cap rotor) and the distributor/coil were redone 2 years ago. I don't know what would cause the coloration on the spark plug.

I did some reading and read that the TPS could be the cause of all of this. I did some testing and the voltage range was 1.2-4.8. With the bottom number being out of the range I went and bought a new one and put it on. This made no difference and also was reading ~1-4.8 so I replaced the original one.

Any thoughts are appreciated.

awg

if you have an OBD2 vehicle and reader, you can live stream the O2 sensor and fuel trim, a bad front O2 sensor can cause issues

ToothMechanic

Here is the data from driving to work this AM. This is after I got to the parking lot and let it sit at idle for a minute.

https://imgur.com/a/vPznbst