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Bucking or hesitating at speed

Weasel

NAXJA Forum User
NAXJA Member
Jeep started hestiating or bucking at speeds from 25+ if held constant at that speed. Thinking the TPS was bad I replaced it and had the dealer adjust it. THye told be at that time my O2 sensor was bad as it was not closing the loop but remaining open. Ok, I had replaced the O2 sensor about three months ago, but I warrentied it anyways and replaced it. Still have the bucking. Took it to the locla shop and they said it was my exhaust manifold leak. I have had a small leak for about 2 years and never any big problems. They said the leak was causing the O2 sensor not to to close therefore running rich.

This however is the exact opposite the Jeep dealer said. They said the leak was there but wasn't big enough to cause problems and didn't sound like it was sucking air in. And the dealer here is very good and I have every reason to trust them.

So I'm wondering what other things might I check that would cause this annoying problem. Should have never replaced the TPS as it didn't seem to be bad, and with 190k on it.

89 4.0...
 
When was the last time you had your fuel filter replaced?
 
I had the exact same prob with my 89@170,000 miles,started replacing sensors and adjusted the tps.Nothing worked so I pulled the motor to find that the timing chain was worn out to the point that the timing gears had 1/4 inch of play in relation to each other(bad for timing)dropped rebuilt motor in anyway and she runs better that she has in the 3+ years I dealt with the bucking...she purrs now without a miss.

Take your cap off and turn the crank pulley both ways....the dist rotor should move immediately when you turn the pulley without any delay.Just something to check and best of all it's free aside from the time you spend checking it....

hth
 
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thanks, I'll check the timing chain. Also I did replace the fuel filter awhile back but it's probably getting close to a year ago.

It's sort of wierd as this happened a few months ago then went away, now it's back and pretty constant. Still don't know why the O2 sensor won't close.
 
Buddy's YJ had the same problem, bucking and stumbling after running at speed for some distance. He replaced the TPS, O2, fuel pressure regulator, fuel filter and nothing helped.Dealer did a fuel injection clean also.

Finally bit the bullet and replaced the fuel pump...problem gone.

BTW...we were on our way out to the Badlands in SD when we started having issues. 02 and filter were done in camp out that way.
 
It happens while cruising in the 1200-2400 RPM range at steady speed, and it's a Renix?

My money is on an EGR problem. Check the valve for proper operation.
 
no idea on the rpm's but yeah it's a Renix. Fuel Pump should be ok but I can check it as well. I think I replaced it about a year ago as well.

How does one check the EGR?
 
The EGR valve allows exhaust gas into the intake manifold to help cool the cylinders under certain operating conditions. This helps maintain proper NOX emissions. It's basic operation is controlled by the computer via the EGR Solenoid, which is located on the driver's side fender near the air box. It has two vacuum lines--one from the intake manifold("source"), and one to the EGR valve ("control"). It also has a connector with wires running back to the ECM. Pulling the control vacuum line loose will disable the EGR valve. Disconnect this line and see if your stubeling stops. Also, you can check to see if the actual EGR valve is stuck. Place your fingers under the EGR valve and squeeze the bottom upwards. It should move fairly easily--there's a spring inside to hold the plunger on it's seat, cutting off exhaust gas flow to the intake. Squeezing allows gas flow, and at idle, doing so should cause the engine to stumble or die. This indicates normal operation.
 
just checked the egr, the solenoid hooked or unhooked dosn't make any difference. I don't see to have any spring on the bottom side or similar to the above mentioned.
 
It's hard to describe, but the spring is inside the EGR body. You would be touching the outside of the diaphragm, which is attached to a rod that goes from the diaphragm to the exhaust manifold. If you reach inside with your finger or a screw driver, you should be able to raise that diaphragm, opening the EGR valve. If the EGR valve is not sealing properly, it can cause stumbling/hesitation.
 
Ok figured out the EGR. When I push the pin towards the engine it runs fine, when I try to pull it out it starts to stumble. When raising the throttle the pins seems to move back and worth a bit and matches up with the hesitations. But when I remove the vacuum lines, it doesn't seem to run any better. So I tried plugging the lines and still the same problem. I even swaped solenoids with one from my 87 and no better. So I'm not sure if the EGR is the problem or not.
 
The fact you can move the diaphragm and get stumbling tells you the EGR is not stuck--this is good. With the control vacuum line to the EGR valve removed, the EGR should not operate--it should not open when the throttle is "goosed". The ECM applies vacuum to the EGR valve's back pressure sensor when the ECM believes the conditions warrant exhaust gas recirculation. If the sensor detects back pressure in the exhaust system, it allows the main EGR valve to open. Some times this back pressure sensor becomes too sensitive and allows the EGR to open too soon (the sensor has an internal spring that determines the amount of back pressure required). I think it's possible for the EGR's pintle, or its seat to become clogged with carbon, allowing some exhaust gas to enter the intake manifold all the time, causing drive ability problems such as you describe.
On the O2 issue, does your ECM go into closed loop? Not sure how to easily determine this on a Renix system (early 4.0 Jeep) , other than if it isn't, you won't pass a Smog check. A leaking exhaust manifold can cause a rich running engine that may keep the ECM from going into closed loop operation. The ECM must be able to control the fuel mixture over a very narrow range, above and below the "sweat" spot. If a condition exists that causes a steady imbalance out side the ECM's range of control, the system won't go into closed loop. Generally, this results in bad gas mileage, increased pollutants, and can kill the catalytic converter. The converter needs to deal with an exhaust gas that constantly switches between lean and rich. Closed loop results in this constant switching. A shop with a exhaust gas analyzer can tell you if your fuel mixture is within specs. If your adventures, do a Google search for oxygen sensor operation and trouble shooting. Then get a digital voltmeter and check out your O2 system. It's really not that hard to do, and there are many online articles to tell you how to do it.
 
Weasel said:
Jeep started hestiating or bucking at speeds from 25+ if held constant at that speed. Thinking the TPS was bad I replaced it and had the dealer adjust it.

A bad TPS only causes the engine to buck during changes in throttle position. The engine won't buck at constant throttle so that's a clue that the problem lies elsewhere.
Your engine has a very high mileage so it's just possible that your timing chain has indeed become overstretched, causing the timing of the spark and injector pulse to get thrown off. Remove the distributor cap and turn the crank with a wrench while watching the rotor. If the crank rotates several degrees before the rotor even moves, you need a new timing set.
You might also want to check the degree rotational play in the distributor shaft. If it's excessive, the cause is most likely to be a worn distributor drive gear. Replacing it is a cheap and easy fix.
 
Well I figured out the problem. Figured I post this just for refrence. Anyways I was told it was my exhaust leak but I had conflicting info form two mechanics I trust. And the leak didn't seem to have gotten any worse. So I checked everything suggested above and it didn't seem to make any difference.

So it really seemed to be a vacuum leak and I had been checking ever single vacuum line. Could find any leaks or unhooked hoses. Then remeber I had move the vacuum canister under the bumper to under the radiator bottle. So I checked it for leaks and it seemed fine. Then I remeber I didn't know which hoses went on which side so the I swaped sides and whatya know. It makes a difference of which side the hoses go on. Seems fixed now, and runs good again.
 
I for one appreciate your informing the forum of the resolution of your problem. Most threads are left hanging after tons of "did you---?". Thanks for the followup.
 
Yeah well I thought I fixed the problem. Came right back and took me a few days but one of the lines I pluged was leaking. The plastic plug was too small and split down the side causing the leak. So I siliconed a screw into the fitting and it's running good again.
 
Hopefully this thread will be very useful in the archives. XJBUBBA gave the best, easiest to understand troubleshoots on a Renix EGR that I have ever read or heard in person. I know how the renix system works and I dont know if I could have put the EGR diagnoses in to words so clearly, And yes Dr. Dyno if your EGR is as screwed up as mine was with carbon it can hesitate at constant and multiple speeds. (The EGR was the only sensor that was a real problem for me to hunt down when I was asking questions becouse no one was pointing me in that direction.) Now If someone has hesitation its the first place I point them too. YMMV, Good Luck
 
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