zj "choking itself"????

[xjennyj]

Having problems with a 5 spd, 4.0L ZJ-GC. It started having problems with maintaining power going up hill- to having problems at any time under load. Also, recently added to that list, it is hard to start, and when it does, (from a completely cold state) the rpms jump up to around 2500 or so, but goes back down after a few seconds. It ultimately appears to be choking itself.

Ruled out:
Most sensors were tested with an ohmmeter or voltmeter per Hayne’s.
Catalytic converter and exhaust (removed)
Fuel pump was tested, although slightly low, not enough to inhibit performance as much as it has
Fuel was replaced, fuel injectors cleaned (cleaner added)
Coil was replaced recently, but tested fine.
Spark plugs, wires, etc replaced.

Major confusing issue: will idle fine, only appears to be “choking” itself under load and NO CHECK ENGINE LIGHT???? Shouldn’t this be on? Maybe the pcm?

 

[bajacalal]

You have a 5 speed ZJ!? :cheers:

How many miles? How 'bout that fuel and air filter? More info is needed. Try searching here for "camshaft position sensor". Also "throttle position sensor" Its bogging under load. Would you be able to tell the difference between if it was running lean or rich? Does it bog then go, or go, then bog down, then go again. Someone recently mentioned the same symptoms, a loose distributor. Could be a number of things but those two are common. Also, the fuel pressure regulator (search) on the fuel rail?. Have you checked the fuel pressure thoroughly, under high RPM, return lined pinched, etc? Don't assume the injectors are clean just because you added the cleaner, if they are seriously plugged or it might not be enough. They might be leaking (search). Did you test the 02 sensor, is the reading normal or does it lean to the rich or lean voltage? How about that idle control valve, it could be stuck open, have you taken it off and cleaned its area of the throttle body? Don't expect it to throw a code unless the pcm thinks something is wrong. It could be a sensor and the comp doesn't realize the error, its calculating the spark and fuel poorly due to bad info. If its a RENIX system forget about throwing codes.

 

[xjennyj]

It has 215K. The air filter is fine, probably should check the fuel filter. Assumed the sensors are all fine, at least they were throwing the appropriate ohms and/or volts, whatever the Haynes specifications were. Did a fuel line and pressure test..I think it was 30 instead of like 31 (or something insanely close like that). Took the fuel rail off, looked at all of the injectors, fine. Seems to be running rich, but the o2 sensor is disconected, since we thought that that is what could have been causing the troubles before. It will bog, then go again, but only because I shift it down into third gear -sometimes even second, and hammer the throttle, and the pickup is very gradual at best, and it hiccups every damn mile per hour it will give me...

Well, back to the drawing board today. I'll see what new and exciting stuff I can come up with today. Thanks for the info.... :sunshine:

 

[Matthew Currie]

I had a similar symptom a while back with my 95 XJ and it turned out to be the MAP sensor. It didn't throw a code, because the sensor wasn't burned out, but it was apparently sending the wrong information. It bogged down and spat under low vacuum conditions, such as going up a long hill or goosing it ona freeway ramp. No guarantee, but a new MAP helped mine.

 

[bajacalal]

Ok, so if you have the oxygen sensor disconnected its not going to run right anyway, but the MAP sensor on the intake manifold would make sense. It is responsible for adjusting the fuel/air, but only at 80% to WOT under open loop mode. Youre in open loop if the oxygen sensor is disconnected. The TPS sensor is common with these systems especially, not sensing you giving gas, ESPECIALLY if you get it wet. Also like I mentioned the CPS (cam position) or crankshaft position sensor because they do go and can have these symptoms, responsible for some spark timing functions. This is particularly hard to diagnose on the web, as 1.) its an intermittent failure and 2.) you likely have an early 90s XJ since its 5 speed, its a pre-OBD2 computer so diagnosing by code can't be done like in some later systems. My recommendation is to see if you can get a code scanner for your system (non-0bd2 right) and have it while someone drives it under load to read the output.

 

[xjennyj]

Thanks for the replies...keep em coming.....This is a ZJ, not an xj, I think thats what you meant, though....

Ok, yesterday. Reconnected O2 sensor, and replaced idle control sensor, ran great for about 10 minutes, then started the same old, same old-but this time, only around corners. This has got to be something with the fuel, or something that is able to move around...

Still, it seemed better than it had, so we decided to go get some dinner, got in, started it up, and it was absolutely terrible. Backfiring, sputtering, with the throttle floored, it could hardly get up the hill. Turned around, took my xj to dinner, and now here it sits, AGAIN....

Well, we have a spare jeep engine, so we may try the map sensor. Thats what I thought all along, but it did the same sputtering, etc disconnected, and it was throwing the right ohms per the haynes manual. As was the TPS, but at this point, I am going to just replace everything, and stop messing around. Hopefully that works, if not we will probably take off the fuel line, check for a clog, and perhaps drop the tank to see if there is dirt in it.

 

[bajacalal]

Thanks for the replies...keep em coming.....This is a ZJ, not an xj, I think thats what you meant, though....

Ok, yesterday. Reconnected O2 sensor, and replaced idle control sensor, ran great for about 10 minutes, then started the same old, same old-but this time, only around corners. This has got to be something with the fuel, or something that is able to move around...

Still, it seemed better than it had, so we decided to go get some dinner, got in, started it up, and it was absolutely terrible. Backfiring, sputtering, with the throttle floored, it could hardly get up the hill. Turned around, took my xj to dinner, and now here it sits, AGAIN....

Well, we have a spare jeep engine, so we may try the map sensor. Thats what I thought all along, but it did the same sputtering, etc disconnected, and it was throwing the right ohms per the haynes manual. As was the TPS, but at this point, I am going to just replace everything, and stop messing around. Hopefully that works, if not we will probably take off the fuel line, check for a clog, and perhaps drop the tank to see if there is dirt in it.

Based on your information I am looking at the fuel pump. Don't know why it diddn't occur to me, maybe because you tested it. Use the spare sensors from the other engine to check, by all means. BTW it is the idle air control valve not sensor, it regulates the idle speed by closing or opening. If stuck can result in bad fuel/air mix. But your story sounds like typical fuel pump(ever own a chevy s10) it stops providing pressure when cornering, hills, does not provide enough flow for high demand when the injectors are really spraying but idles fine. You say you tested it but you tested it at idle pressure right. They are electrical and have a habit of failing intermittently, after movement occurs. The intermittent failure progresses into a full blown failure. If its 215k on the factory pump its nearing retirement. And if its a fuel pump, I bet you'll never get a check engine light as the pressure is not monitored. How does it behave when the tank is completely full?

 

[xjennyj]

Checked all of the sensors...nada. I don't feel all that bad, though, since we can rule those out. Got a new O2 sensor, too, while we were at it.

We are thinking fuel pump for sure. But I have a question. We disconnected the fuel line to see if it was pumping gas when he turned the key, and it did, but only occasionally. Sometimes, there was more gas pumping out than other times... what do you think?

 

[Matthew Currie]

Checked all of the sensors...nada. I don't feel all that bad, though, since we can rule those out. Got a new O2 sensor, too, while we were at it.

We are thinking fuel pump for sure. But I have a question. We disconnected the fuel line to see if it was pumping gas when he turned the key, and it did, but only occasionally. Sometimes, there was more gas pumping out than other times... what do you think?

It should be steady when it's running, but if it's like the XJ, it will cycle off a few seconds after you turn the key, until it actually starts. If there's some doubt, you should probably connect the pump directly, or hotwire the pump relay, to make sure it's the pump, not the control circuitry. Assuming it is, I would be looking at the fuel pump pickup as well as the pump itself. I don't know just how it's arranged in a ZJ, whether it has an in-tank filter, or a sock, or what, but it sounds as if it's not picking up fuel. That could be a bad pump losing its prime because it can't pull hard enough, or a clogged/cracked/ pinched/???'ed pickup line or filter. My guess is that whatever it is you're going to have to take the pump out and eyeball it.