anyone seen this?? help...

I've got a strange problem with my 89 XJ 4.0:

About a hundred miles into a tank of gas it started stumbling/surging at idle and at cruising speed, but not during hard acceleration or at idle in neutral. At first I thought it was bad gas (we've had a lot of rain lately, and who knows when the tanker filled the ground tanks at the station...). It doesn't happen 100% of the time, about 50%.

I ran Berrymans, Sea Foam, and then ethanol at max concentrations, but nothing helped. I then drained the tank through the fuel rail (until air was found). The gas didn't seem to have water in it (clear container test), but maybe it was just "bad", even thought it was BP. I put in another major brand. It still does the same thing.

It stumbles on light accel, idle in drive, and cruising at highway speeds. WOT, hard accel, and decel seems fine. I disconnected the O2 sensor to force it into open loop mode, but no diff. The fuel pump pumps fine, and I disconnected the vacuum line to increase the pressure to test. No diff. If it were pressure or delivery, then I should get bad performance on WOT, but I get just the opposite...

It starts fine, and I keep it running at idle by using my foot when needed. I had long since manually adjusted the idle stepper to manually override idle speed. Rock solid until now. Something has gone wrong, and of course it happens at cruising highway speed too, so it isn't an idle circuit issue.

Any ideas what it could be? It's intermittent, so it's tough to be under the hood to find at the same time. I think I've ruled out gas, and it seems to be opposite a fuel delivery problem. It could be an ignition problem, but any pointers on why I have it only some time, and only at light usage, not WOT or heavy accel?

Thanks in advance,
Skip

Doesn't sound like ignition, which would more commonly cause problems under load and hard acceleration and behave fine at idle. Poor fuel supply would more likely cause stumbling under load, and work better at idle, too. It sounds as if the problem occurs particularly when vacuum is high. How about EGR? On this vintage, the egr valve will sometimes stick half-open. Try flicking the diaphragm with your finger and see if it pops closed. I'd check vacuum all around too. I wonder if a bad MAP sensor would do this, too. Last time my MAP misbehaved it was under low vacuum conditions, rather than high, but it is one of the things here that is vacuum dependent, so I'd consider it a suspect.

Finally, of course, since this is a Renix, you might suspect the TPS, and also should probably go around and unplug/replug/clean every electrical connector within reach, since Renixes (or are they Renices?) seem especially sensitive to poor contacts.

I would be qualifying the TPS and vacuum systems including the EGR valve. Ignition/spark trouble will usually show-up more under WOT condition so my guess is that the TPS has a bad spot and the ECU simply doesn't know what to do at that moment.

The indivivdual injector connectors often hold water/moisture and become very intermittent on my rig. Act's like a bad plug wire and misses.

An update:

When accelerating, I can change throttle position and it stumbles over a fairly wide range when stumbling, until it gets to hard accel. At idle, it will keep trying to stumble, even with me changing the idle speed. It may not be the TPS.

I started it today, and let it idle in N for a while. As usual, it stalled once (if left in N long enough, it'll do it there too, but dropping into N at a light will usually keep it going) without ever having touched the pedal. I played with the pedal and at about 1200 rpm it would smooth out again. Towards the end, it would idle OK, but 'seemed' to vibrate a little bit more than usual (a rough idle, but still smoother than most older cars, so I can't just point to it a tell a stranger it's got a rough idle

I also _think_ I noticed an odd smell from the exhaust a couple of times when it was really rough, I was idling, and the air was still. Other times, no.

I decided to take it for a drive, then check it out again. Well, it stumbled a few times, then ran fine! I hestitate to say, but I _think_ it would stumble as I went over bumps in the road too. I tried it one time, on a road with bits of construction going on and sections of metal road plates. Going slow, engine at idle, it would stumble each time it hit a bump in the road.

Thinking it could be a loose connection, I tapped on every wire, connector, injector, etc., I could find. Not a single miss! I idled in N and drive for over an hour and not a single problem while I had the hood up and looking. Drove it and no problem.

I've been filling only 5-7 gal at a time since draining the tank (in case I have to do it again), and I'm on the second 7 gal now.

I hadn't driven in the rain, or gone through water, before or since the problem started. Even though it's rained a lot, I wasn't out in it. Distributor is dry.

Thanks for the ideas. If this brings up any other ones, let me know. Now I don't know if it's 'trustworthy' now...

Skip

Check your knock sensor, its about 6 to 8 inches in front of the O2 sensor just above the lip of the oil pan threaded into the block.

Part of what you describe, especially the dying on bumps and such, were caused by a bad one on mine, retarding the timing so much it killed/stumbled the motor.

The most common thing is the inconsistency...

Cory

I still think a possibility is that it's a bad egr valve. This happened on my ex-wife's 88 long ago - internal wear caused it to hang open intermittently, but sometimes it would snap shut.

You can reach around the underside of the EGR valve and put your finger on the diaphragm. Next time it happens, try pushing it open and then letting it snap shut. If the problem goes away, you've got your answer.

The knock sensor idea is also interesting. I noticed on my old 87 that when the tranny mount went bad and the exhaust knocked hard, it messed up my idle, and I think that was caused by the knock sensor picking it up.

Take a look at the CPS wiring. Lots of people have had the wire melt through and short out against the exhaust manifold. Mine has done this a couple of times. When you drive over bumps, it makes the wire tap the exhaust manifold and it shorts out. Really easy to look at with a flashlight.

Wow, those are all good suggestions. The only thing I might add is to start eliminating things. I would first disconnect the vacuum line to the EGR valve, and see if the idle improves. (Don't forget to plug the vacuum hose).

Once the EGR is confirmed or eliminated, go the the knock sensor next. Maybe simply unplugging it would tell you something. Same with the MAP sensor. Good luck.

Doubtful on the knock sensor. I ran mine for months without one and the only change was a little ping when hot under low RPM load.

When you mentioned RR tracks that reminded me of my CPS connector. We'd drive over bumps and tracks, sometimes it would die. But it never caused rough engine operation, it simply died and then crank but no start. Re-seated the connector behind the valve cover and it would go again. I finally spliced it out-of-circuit and never had a problem since.

I'm wondering about your fuel system? How much will it pump in 30 seconds? Should be at least a quart. Pickup tube might be bad in the tank and causing it to suck air?

Throw an analog ohm meter on the TPS and check it for smooth resistance swing across the range. If an auto TPS, check both pots/wipers. Probably not the problem but you asked and it's a major contributor to ECU information and a "wear" sensor.

Also, check and qualify every ground from engine - frame - battery. Sounds like a good possibility something could be amiss there too.

Could be BAD GAS or water in gas. It DOES happen. Wait till your next fillup and load it with some 93+ octane and you'll find out quick.

It might have been bad gas. A few stations around here got shut down for water in gas, but not the one I went to. Anyway, I added Berrymans and Sea Foam to the problem tankfull and it lessened it. Ran it down to a couple gallons and added 5 gal 93 octane - no help. Added ethanol to the tank - worse. Pumped the tank out through the rail and added 5 gal regular from another source - got better, but not fixed.

Ran it down and added 10 gal regular. After a few miles it went away and has been solid ever since. Added Marvel Mystery oil to the tank to lube any water exposed parts like the injectors and pump. Runs great again.

I may add some of the other additives a few more times to take up any residual water in the bottom of the tank if there. The stumble on bumps was curious though... Maybe it was trying to knock on the poor gas and the bumps just set it over. All the wiring is fine and not burned.

BTW, I used to have the CPS starting problem until I took some Ox-gard on a toothpick and ran some around in the connector contacts. Fine ever since.

Thanks for all the help. I'll write back if it goes bad again.:repair:

Any suggestions on what to do with 4 gal of "bad gas"? It actually looks like green Gatorade when in a clear jug...

This may be a stupid question but have you check or change you plugs. I put new plugs in my 87 and had almost identical problems. It would stubble all over itself when I mashed the accelerator until a higher rpm was reach and then it would clear up and accelerate hard. The Number six plug was dead at lower RPM and would only fire when the engine speed increased enough. This may not be your problem but hate to over look the simple items (I spent a week chasing this down but though no way the plugs are bad they are brand new). Good luck and let us know what you find.

Checking plugs are on my list, but probably not this issue. Even at lower RPMs, a more open throttle (richer condition) solved things each time.

It's been several days since the 3rd tank of gas and no problems. I have to conclude it was something in the gas.

I'm going to try to get rid of anything that may be sitting in the bottom below the pickup tube in the tank. It's been rock solid again though. Whatever it was, it was nasty, as it took getting down to a couple gallons (approx - below empty on the gauge) and refilling 3 times to get it right again.

You can always spot genious, it thinks just like you do :laugh3:
You guys sure enough hit many of the high spots.
This thread would be a good start. for a new to me Renix check list.
You could sticky it.

Coop there wasn´t a thing wrong with your suggestion, seen the same thing before. A quick look at a couple of center, plugs, will tell you rich, middle, lean, right quick and you can compare it to yout last drive, slow middle or interstate and jump to some obviuos conclusions.
In place of the knock sensor (though I have seen them hanging by the wires before), I have a close look at the harness, from the front of the fuel rail, down the front of the motor and under to the knock sensor, ECU/engine temp. sender and the O2 sensor.
Most every old Renix I´ve looked at had, at the minimum a rub, on the harness or wires, some were cooked.