Hesitates apon acceleration

[Tracy92]

I have this problem with my XJ, it's a 92 4.0 with a standard and about 354,000kms on the odo. The engine idles fine but hesitates slightly when you give it a shot of gas. On the highway it's almost not noticeable. In town, it progressively get's real bad. It hesitates, stumbles and occasionally backfires. It's really wierd because if you hold the gas peddle to the floor for 10sec or so, it will run through the symptoms then out of nowhere, the problem disappears and it accelerates fine till you shift gears, the problem returns through every gear while your accelerating though. If your exceptionally easy on the gas pedel it doesn't really do it, thats REALLY light on the pedal though. Checked for diagnostic codes. No codes present.

New parts list:

TPS (aftermarket)
plugs
wires
cap and rotor
air filter
coolant sensor
o2 sensor
Sending unit, fuel pump and filter

fuel pressure check OK
performed injector balance test. Injectors all evenly balanced

I had it at a local shop and he put it on the computer and couldn't find any problems but started replacing likely parts. He had it for quite a while trying to work on it during slow periods and save me money but after a couple months wasn't getting anywhere. He did say that if he unplugged the coolent sensor it would run better but be alot worse on gas. He has suggested changing the MAP and pickup sensor in the distributor.

So do any of you guys have any suggestions? Any similar problems that you found fixes for?

Thanks.

 

[BBeach]

My friend just had this problem with his civic, or at least a problem similar to this. It ended up being a cracked distributor cap, we replaced that and all was well. But apparently you already checked that so im not sure. Is the exhaust system fine? Maybe a clogged cat, idk.

 

[BBeach]

Also try looking through some of the threads you'll see at the bottom of this page saaying "similar threads".

 

[Fish'nCarz]

Have you checked compression in all cylinders? Does the exhaust smoke alot when it does this, and what color is the smoke if present?

I'd run a can of sea-foam through the gas and clean the throttle body next if it was mine. With that many miles it might be accumulated gunk.

 

[J-Roc]

Clean out your Throttle body!
It's the cheapest thing to do first, a can of TB Cleaner is like $7
Clean all the carbon deposits out of it, remove the IAC Motor I bet you any money it is all gummed up. I had the same symptoms you describe, turns out my IAC was all gummed up and was stuck closed, damn thing nearly fell flat on her face off the line.

 

[Blaine B.]

Pop a squirt of WD40 into the IAC too, after cleaning. Down the shaft that is, where the spring is.

 

[Ecomike]

I have this problem with my XJ, it's a 92 4.0 with a standard and about 354,000kms on the odo. The engine idles fine but hesitates slightly when you give it a shot of gas. On the highway it's almost not noticeable. In town, it progressively get's real bad. It hesitates, stumbles and occasionally backfires. It's really wierd because if you hold the gas peddle to the floor for 10sec or so, it will run through the symptoms then out of nowhere, the problem disappears and it accelerates fine till you shift gears, the problem returns through every gear while your accelerating though. If your exceptionally easy on the gas pedel it doesn't really do it, thats REALLY light on the pedal though. Checked for diagnostic codes. No codes present.

New parts list:

TPS (aftermarket)
plugs
wires
cap and rotor
air filter
coolant sensor
o2 sensor
Sending unit, fuel pump and filter

fuel pressure check OK
performed injector balance test. Injectors all evenly balanced

I had it at a local shop and he put it on the computer and couldn't find any problems but started replacing likely parts. He had it for quite a while trying to work on it during slow periods and save me money but after a couple months wasn't getting anywhere. He did say that if he unplugged the coolent sensor it would run better but be alot worse on gas. He has suggested changing the MAP and pickup sensor in the distributor.

So do any of you guys have any suggestions? Any similar problems that you found fixes for?

Thanks.

You will need to search for and find the how to test and replacement details but here is a list of what I would test:

The distributor (Several parts, stator wire*, stator, shaft play!! could be causing timing errors) consider replacing it with a rebuilt. Mine was toast at 220,000 miles. (high probability.....)
The EGR valve and the solenoid that triggers it, (if the 92's had an EGR valve?) high probability a bad EGR would cause your symptoms
Check every where for any trace of a vacuum leak
High volatge coil (low voltage or a leak)
Ignition control modual (ICM) under the high voltage coil (Haynes manual has some tests*)
Air temperature sensor in the air intake manifold (high probability*)
Loose connections* anywhere in the sensor/ECU/TCU/ICM wiring harness system!
You might also consider testing the MAP sensor*.
Lastly, check that the TPS and O2 sensor connectors are not attached backwards to the wiring harness connectors (a tip from my own personal experience)

A lot of the above testing can be done with an inexpesnive, <$10.00 volt/ohm meter (* devices that can be tested partially with a volt ohm meter).

Mike McGinness
South Houston, TX

 

[Tracy92]

I tried unplugging the coolent sensor like my local mechanic told me, but found the vehicle emediatly runing through all the symptoms, and became totaly undrivable.

I cleand out the throttle body and noticed some improvment but the symptoms are still there. I was toled from the local mechanic that he found no vacume leaks on the vehicle.

 

[Fish'nCarz]

I'd still try the Sea Foam. In the gas tank and down the throttle body. Do a search and you'll find several posts describing how to do it. Cost is only about $5.00 (USD) and it might save you a lot in time, frustration and $$$.

FWIW

 

[Ecomike]

Find a mechanic that knows how to use an ohm meter! Most of the high probablity stuff I listed below can be tested with an ohm meter with out removing or replacing the part!

Your test implies that the coolant temperature sensor is probably working OK, but an ohm meter will verify it. Also the coolant temperature does not change quickly.

Your problem has to be caused by something that changes in a split second, like loose connections when the engine moves while being accelerated, or the intake air temperature (sensor), during acceleration the air rapidly cools for a moment, or the distributor when torqued during acceleration if the bearings are bad or the stator is bad it will cause your problem, or the EGR valve (but I am not sure if they used them after 1991, does anyone know if the 92 had an EGR valve?)

Try replacing the air temperature sensor in the air intake manifold, but it can be easily tested with an ohm meter first!!!!!!

Mike McGinness
South Houston, TX

 

[Spanky414]

This is a friend of Tracy92s thanks for all the help guys.

We were driving it around today and I thought I would point out that while driving he can floor it and it stumbles and has low power but does pick up speed and the RMPs go up, then once it gets up in the revs all of a sudden it will smoothout and really start to pull (as in the power comes back). Then he shifts and has to do it all over again. It also doesn't seem to act up as much when he takes it really easy on the pedal.

We tested the air and coolent temp sensors and they seem good.

This weekend we are swapping the cluster out for one with gauges and changing the temp sender in the back of the head and oil pressure sender so he knows whats really going on with his engine. Idiot lights really don't help much when trying to diagnose problems.

I'm not sure about the EGR valve, I don't think so but will check again tommorow.

 

[Spanky414]

Problem seems to be solved, my friend took it into an electrical specialist and he concluded that the fuel pressure was too low and replaced the pump.

The original mech said that it was low but within spec, 28-29 lbs. The new mech checked and it was at 18-20 lbs.

Who knows why there is a difference, maybe the first guy screwed up or maybe it was getting worse. The old one was a carter aftermarket pump that had replaced a noisy original.

Now we realize that he replaced the noisy fuel pump and it started acting up, we should have check that pump right away. Of course we trusted the mech at the first shop.

The new pump is a delphi DPFE0108.

So anyone having similar problems, check your fuel pump and system.

 

[papacan]

Problem seems to be solved, my friend took it into an electrical specialist and he concluded that the fuel pressure was too low and replaced the pump.

The original mech said that it was low but within spec, 28-29 lbs. The new mech checked and it was at 18-20 lbs.

Who knows why there is a difference, maybe the first guy screwed up or maybe it was getting worse. The old one was a carter aftermarket pump that had replaced a noisy original.

Now we realize that he replaced the noisy fuel pump and it started acting up, we should have check that pump right away. Of course we trusted the mech at the first shop.

The new pump is a delphi DPFE0108.

So anyone having similar problems, check your fuel pump and system.

In defense of mech 1,you cain't diagnose something thats not there!

 

[Spanky414]

In defense of mech 1,you cain't diagnose something thats not there!

? Did I somehow make it sound like there was no pump in it?

There was a fuel pump in it, it was driven to his shop. He tested it and said it was a bit low but still good. He was infact wrong and in my opinion doesn't know jeeps as well as he says he does.

 

[Ecomike]

? Did I somehow make it sound like there was no pump in it?

There was a fuel pump in it, it was driven to his shop. He tested it and said it was a bit low but still good. He was infact wrong and in my opinion doesn't know jeeps as well as he says he does.

I think Papacan was just trying to say that the fuel pump pressure problem may not have been there when mechanic 1 checked it. Problems can come and go. They can be very erractic and hard to isolate no matter how experienced and knowledgable one is. Also pressure gauges can go bad. Hell mechanic number 2 could of had a bad gauge, replaced the fuel pump for the wrong reason and had it work (solve the problem) simply because the old pump was not delivering enough flow instead of not delivering enough pressure. Fuel pumps can deliver enough pressure at idle, but not enough flow at higher loads. There is a seperate flow test to test for flow.

Anyway, S#@t happens. Glad to hear to you got the problem solved!

 

[Spanky414]

Ah, either way, even the original check wasn't within specs, thats why I don't trust the first mech.

And of course, just my friends luck, the problem is back but not as bad after a couple days of driving.

 

[Ecomike]

Ah, either way, even the original check wasn't within specs, thats why I don't trust the first mech.

And of course, just my friends luck, the problem is back but not as bad after a couple days of driving.

Well that confirms the diagnosis, your friend definately has a genuine JEEP

There is a check valve in the fuel pump assy. that could be causing trouble too if it is leaking fuel under pressure back into the fuel tank.

I looked over this thread again and I do not see any mention of testing or results of tests, or even suggestions to test the fuel pressure regulator! I would also check that all the vacuum lines are tight with no leaks. You should get 31 psi at idle with a working vacuum operated fuel pressure regulator on the fuel rail inlet, then 39 psi if you disconect the vacuum line from the fuel pressure regulator and plug that vacuum line during the test, that is with a properly working fuel pump and fuel pump check valve, fuel filter and good fuel lines (no kinks in the lines....)

 

[Spanky414]

I'm hesitant to even post anymore in this thread because I'm getting all the info 2nd hand and not directly from the mech. I've just now found out that fuel pump the 2nd mechanic removed was not infact a carter but a delphi, exactly the same as the one that replaced it, and that was has gone bad also.

My friend asked about the fuel pressure regulator and vacuum lines (as me and a another friend prompted him to do) and the mech said that he tested the pressure before the regulator and found it too low. Doesn't seem he has actually tested fuel rail pressure which would tell him if the regulator is working right. Who knows though, like I said I'm getting this all 2nd hand.