Stroker stumbles from 1.5-2.5k

[seanyb505]

Built about a year ago and ran fine for the first 4-5k miles. Before the stroker I had an issue where I could drive it normally, turn it off, and it would stumble after I shifted to second. The stumble would go away as soon as I let off the gas(shifting into thrid for example). I never looked into fixing it. Now the stroker does the same thing, only much worse and all the time. It doesnt matter if its cold or warm. As soon as I get through that RPM range it takes off like normal and will run fine until I have to accelerate through the range again. If I try to hold an rpm at idle it will fluctuate about 100 rpms and slowly fall. I disconnected the battery and held the key for 30 seconds, and it seemed to go away for the first 2 minutes then was back again. The o2 sensor has never been changed so I have one of those ready to go. I suspected maybe the fuel filter, but the auto part stores dont have it. Can you change only the fuel filter, or do you have to do the whole pump? Any other ideas?

[Muad'Dib]

What year?

Sounds like you have a lean spot. I had a similar problem a long time ago. It was the fuel pump going out.

When you said filter were you talking about the filter screen for the pump or the actual fuel filter? I know for my year xj (87-90) you can get the fuel pump "strainer" separately or with a pump. I just looked up a 97 at rockauto and you can get a strainer separately for that year also... so i don't see why you wouldn't be able to get one.

A clogged fuel filter could do that especially if you have never changed it out.

Try a junkyard fuel pump and see what happens. Worked great for me. However if you have a newer XJ, the tank will have to be pulled which kind of sucks... but still isn't that hard.

[SilverXJ]

You can get the filter separately for the 97+. Also, i think there may actually be another filter inside the pump assembly.

[seanyb505]

I put in a new upstream o2 sensor, spark plugs and patched some wiring. Still stumbled. Then I threw on a TPS and another TB sensor from an 01 and it still stumbled. It really smelled like gas when I was working on it, so Im guessing its running too rich and I think the injectors I have may be the cuplrit, even though it ran for 4500 miles without a similar issue. They were never tested, so thats next after I get it scanned by advance or someone.

[SilverXJ]

Could the injectors be dirty or some holding open?

When you say scanned, is it throwing an engine light? Advance or Autozone won't tell you anything more than an engine light.

Also, I still think its a sensor based on that it happens at 1.5k to 2k, and it shouldn't be in open loop at that time.

Have you checked your fuel pressure?

Also, a gas smell is normal considering the injectors fire into the manifold and some fuel does catch on the walls.

[seanyb505]

I suppose the next step will be verifying voltage of the sensors, and then perhaps have the injectors tested. The gas smell was stronger than normal, but the richness would only be a symptom and not the problem. The CEL is coming on and turning off at random times. The stumble still hasnt left. I tried premium and it didnt help.

[gradon]

I've read of dead spots on TPSs before--you should monitor the tps% when this happens. Keep your foot in the same position on the pedal and see if the % changes on the scanner.

[seanyb505]

Any ideas for a good cheap scanner? I plan on getting a decent one later when the turbo comes, but I dont want to shell out the money right now.

[seanyb505]

Ive tried some different injectors - from a 95 Trans Am, the kind with 6 holes. Its smoother overall, but it still stumbles. It seems that the stumble is only there when its cold. Once it is up to operating temperature its gone. Even if its cold, once you get past it in a gear its gone as you wind through the gear. If I shift and fall back into the range its there again. So its affected by temp, not by RPM. Any Ideas?

I have yet to try a new fuel pump yet, but with c4c there are waves of XJs coming into the junkyards, so Ill likely grab one this week.

[yuppiexj]

To check for a dead spot in the TPS simply backprobe the leads with a multimeter and slowly cycle the throttle linkage watching for the needle (or display) to drop to infinity or 0 Ohms . (infinity is an open ciricuit 0 Ohms is a dead short in the sensor)

I use Ohms with the battery disconnected, you can use Volts if you want the circuit powered.

I've had 2 TPS sensors go bad right at the highway cruise throttle position. (Both on a RENIX, don't spray water on them)
The symptom on the first one was to drop out of OD to 2nd gear (computer saw WOT/infinite ohms)
The second one was the opposite, drop in power and the torque converter would unlock (computer saw closed throttle/dead short)

It's a 5 minute $0 check (assuming you have a multimeter)

[seanyb505]

Ill check that, but I dont think the engine would react the way it is under described TPS signals. It seems it would either cut out or go wide open like you mentioned.

[seanyb505]

I finally got around to installing a junkyard fuel pump. In the first two times Ive driven it, there has been no stumble. Im hoping it was a combination of the lt1 injectors and the other fuel pump that did it.