PDA

View Full Version : Momentary rough running about 1 minute after a warm start.


xjalfh
September 23rd, 2017, 10:53
98 XJ with 4.0L.

This problem started out as occasional and only at higher temps, but has increased in frequency over the last year or so to where it now happens every time.

It will start fine in any temperature, but when warm (mid 80s or higher ambient, or if engine is still warm from previously running) about 1 minute after starting it will run really, really rough for about 5-15 seconds. Nearly zero power, stumbles and sputters, popping in the exhaust. Going WOT will cause it to fall flat on its face and nearly stall, while pedalling it between 0-50% throttle seems to bring it out of stumbling more quickly. That could just be coincidence, though.

When it comes back out of it, the engine will continue to run just fine until it is shut of and restarted again (although it may be down just a little on power for a minute or so after the stumble).

A bit dangerous, as it's acting up is timed to occur just as you'd be pulling out into traffic from a parking lot for example.

No codes. I've done some shotgunning. Throttle body cleaned. IAC was cleaned, then replaced. TPS replaced. Exhaust, including O2 sensors were new 6 years ago, but I've since replaced both O2 sensors again (NGK). Front one was completely covered in carbon. Understandable, I suppose, as it tends to pop in the exhaust when it is acting up (fuel, but no spark? excessive fuel with or without spark?).

This is a WAG, but the timing of the problem seems to possibly indicate something going funky when the system is switching from an open-loop starting mode to a closed-loop running mode. I honestly don't even know if/when that occurs.

I've found numerous threads on rough running being caused by a number of issues, but nothing this temporary.

Any suggestions?

Thanks.

trippled
September 23rd, 2017, 14:24
Man as soon as started reading I was gonna say the o2 as the woman's 99 did just that but on a cold start and a new o2 fixed it. Any access to live data? With no codes I think that's going to be pretty critical. Checked fuel pressure during the concern?

md21722
September 23rd, 2017, 16:53
The system goes open loop under many conditions such as heavy throttle. I would consider checking or replacing the crank sensor and don't rule out a bad coil, in addition to what Trippled said.

Tim_MN
September 23rd, 2017, 17:19
Are all of the tune-up parts fresh ?

What are your most recent gas MPG's, and how do they related to your historical MPG's ?

The symptoms described are not typical CPS failure symptoms, cross it off the list. The coil should be tested, but the symptoms are not typical coil failure symptoms.

I would start with fuel pressure testing, being sure to closely monitor the pressure while the symptoms occur. Leave the pressure tester in-line and test for fuel pump check valve failure also. I would test the Coolant Temperature Sensor also.

The onset of the symptoms really does suggest an issue when the PCM switches from cold start open loop to closed loop operation. Bucking, backfiring, and stalling should make O2 sensor issues the first thing to suspect. Check the O2 sensor heater fuses, visually inspect the O2 sensor wire harness for damaged wires and chafed/melted wire insulation, inspect the O2 sensor plugs for corrosion and pushed back wire pins, and test the O2 sensor wires for continuity. If the O2 sensors were not practically new, I would have suggested testing them as well.

Green XJ Jeep
September 23rd, 2017, 19:01
I would suspect the temp sensor

xjalfh
September 23rd, 2017, 20:08
I got the vehicle about 6 years ago. At that time I replaced the coil, cap, wires, plugs. So that's all the fresher the tuneup parts are.

I replaced the fuel pump module about 4 months ago (Bosch) due to hard starts (check valve). Replacing the pump didn't seem to have any affect good or bad on the problem at hand at that time.

I'll have to find me a fuel pressure gauge anyway.

I now have access to an odb scanner that does recording, but of course it rained this afternoon so it was too cool and it didn't act up. I tried recording anyway just to test it out and the scan tool appears to only record a set of readings every 5 seconds. It may end up being useless for this problem. I'll give it a shot tomorrow though.

There aren't any readily apparent problems with the O2 wiring harnesses, and the heater fuse is good. I'll have to dig more into the O2 harness. About the only thing I've done as far as wiring is concerned is to clean, and reseat the connectors on the PCM.

If I'm lucky, I'll be able to capture some obviously funky readings tomorrow, maybe temps or O2 readings, or???, but at one reading every 5 seconds it might be difficult.

Any recommendations for an obdii device, even one that requires a laptop, or smart phone, that can do more continuous recording?

HobHayward
September 24th, 2017, 17:52
A bad rotor/cap led to stumbling right after a warm start for me. Otherwise it ran pretty much fine. It was pretty weird.

The1AndOnlyBug
September 24th, 2017, 18:42
98 XJ with 4.0L.

This problem started out as occasional and only at higher temps, but has increased in frequency over the last year or so to where it now happens every time.

It will start fine in any temperature, but when warm (mid 80s or higher ambient, or if engine is still warm from previously running) about 1 minute after starting it will run really, really rough for about 5-15 seconds. Nearly zero power, stumbles and sputters, popping in the exhaust. Going WOT will cause it to fall flat on its face and nearly stall, while pedalling it between 0-50% throttle seems to bring it out of stumbling more quickly. That could just be coincidence, though.

When it comes back out of it, the engine will continue to run just fine until it is shut of and restarted again (although it may be down just a little on power for a minute or so after the stumble).

A bit dangerous, as it's acting up is timed to occur just as you'd be pulling out into traffic from a parking lot for example.

No codes. I've done some shotgunning. Throttle body cleaned. IAC was cleaned, then replaced. TPS replaced. Exhaust, including O2 sensors were new 6 years ago, but I've since replaced both O2 sensors again (NGK). Front one was completely covered in carbon. Understandable, I suppose, as it tends to pop in the exhaust when it is acting up (fuel, but no spark? excessive fuel with or without spark?).

This is a WAG, but the timing of the problem seems to possibly indicate something going funky when the system is switching from an open-loop starting mode to a closed-loop running mode. I honestly don't even know if/when that occurs.

I've found numerous threads on rough running being caused by a number of issues, but nothing this temporary.

Any suggestions?

Thanks.
IMO the highlighted to me sounds like heatsoak.

xjalfh
September 25th, 2017, 18:25
I had a fairly lengthy reply that somehow never made it to the thread.

Anyway, thanks for the input.

The cap/rotor/coil/wires were all replaced when I bought the xj about 5 years ago. Strong white spark and the parts all look good.

Sensor heater fuse is good, and the wiring shows no apparent damage. I'll definitely have to look at the O2 wiring much more closely. Squirrels did do a number on the cooling fan wiring once. I remember it throwing a code last winter when it was just idling to warm up. It was related to the front O2 sensor but I don't remember the exact code, and once it was cleared it never returned.

The odb scanner I have does have a record function, but it only records readings every 5 seconds. I ordered a new one that connects to a laptop/phone, so hopefully I'll capture something helpful in the next few days.

smac999
October 6th, 2017, 02:37
my 99 was doing exact same thing a few years ago. 02 fixed it

smac999
October 6th, 2017, 02:39
IMO the highlighted to me sounds like heatsoak.

heat soak causes no start or bad running right at start.

cruiser54
October 7th, 2017, 07:21
I wonder if you have an injector or 2 that leaks gas into the cylinders after turning it off.

A fuel pressure test would help verify this.

xjalfh
October 14th, 2017, 14:28
Well, I finally found a little time to dig into it deeper. It set a P0135 code (I'm sure that's the one it set last winter.) so I started with the upstream O2 heater circuit.

Rechecked the fuse for the heaters and it was good, with 14V on both sides when running.

14V at the O2 sensor connector on the feed side, but 14V on what should be the ground side as well. No voltage drop across the heater. So, it seems there's a bad ground.

Indeed, ohmmeter verifies open circuit on the ground side of the connector.

Looking at the wiring diagram, it goes to S107 then to G101. The ground tests good and was clean, but I cleaned it anyway.

I am still digging for S107 though. It seems that nearly everything grounds through the S107 splice, so I would think it would have to be quite the bundle of grounds. The manual shows that it should be inside the harness at the rear of the engine on pass. side. Still trying to find it.

Does anyone know where the grounding splice S107 is located?

xjalfh
October 14th, 2017, 17:41
Gave up on looking for the splice that was supposed to be in its ground circuit, and backtracked from the connector on the O2 sensor. Should have started there in the first place instead of trying to work from the ground, backwards.

There was no apparent damage on the outside of the loom cover, but inside I found the wires had been pinched at some time and the ground wire completely fell apart when I took it out of the loom.

Don't know if it was just barely hanging on by a thread that it didn't always set a P0135 code (doubtful), or the heater hasn't worked for ages but conditions were just not right for setting a code until now when the lower temps start returning.

Now to redo the wire looms and tape I unnecessarily removed and hope it runs like a champ again.