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Jer91 05-07-2013 08:53 AM

I have a 1991 jeep wrangler yj. Lately I have had problems with the engine hesitating. The shop has installed a new fuel pump and I installed a new fuel filter but this didn't fix the problem. The hesitation only happens when the engine has been running a while and is warm. It feels like the engine just loses all power for a few seconds. If I smash the pedal down to the floor it will regain power but if I hold the pedal steady it will not regain power. What could be the problem? Any help and advise would be greatly appreciated.

Squirrel 442 05-07-2013 09:02 AM

Give a little more details such as engine size, miles, how old are the plugs/cap/rotor? Have you given it a tune up?

Could also be vac leak or engine timing.

Did the shop you took it to check any of those?

Jer91 05-07-2013 11:48 AM

It's a 1991 yj 4.0 inline 6. It had a full engine tune up about 2 months ago. The shop says there is a bit of oil blowing over into the air cleaner as well as the head gasket has a small leak I believe. As for plugs etc. I have no idea how old they are.

Jer91 05-07-2013 11:49 AM

It also has 103,000 miles

Squirrel 442 05-07-2013 04:36 PM

Do the plugs look two months old (clean white) or dirty on the outside. If the shop did a full tune up the plugs should have been changed.

What does your receipt say they did and how much did they charge you? Just trying to see if they are taking advantage of you or if we have a different view of what a tune up involves.

How much mechanic knowledge/wrenching skill do you have?

Edit: Did they or you see if you had any CEL codes?

Jer91 05-07-2013 10:27 PM

I will have to check on that information and look at the receipt they gave me. I can't remember off the top of my head. The plugs look fairly new. I have a good bit of mechanical knowledge dealing with working on motorcycles but workin on the jeep is a pretty new area for me.

Squirrel 442 05-07-2013 10:42 PM

You'll be fine then, just wanted to make sure I explained things so you understood them.

These Jeeps are sensors on wheels. See if you are showing any codes. Turn the ignition on then off three times. On the forth time leave it in the on position.

The check engine light will start to blink. Have a pen and paper ready. The codes will be in two digit numbers. so a code 23 will be flash, flash ........flash, flash flash.

You may have many codes (example 12, 33, 54, 55). At the end of the code sequence you will get a code 55 (that means "the end") Post up the codes and we will help from there.

Jer91 05-08-2013 06:31 AM

Ok great I will check that this morning when I get some free time

Jer91 05-08-2013 08:00 AM

The codes I got were, 12, 33, 21. I was reading online and from what I researched the only one I should be worried about is the 21 which is o2 sensor. Is that right? And would that cause a hesitation of the engine?

Jer91 05-08-2013 10:05 AM

I looked at my exhaust between the headers and the o2 sensor and there is a large crack running around the one pipe. Could this be allowing extra oxygen in and messing of the fuel to oxygen ratio of the jeep? Could this make it bog?

Squirrel 442 05-08-2013 03:38 PM


Originally Posted by Jer91 (Post 3733221)
I looked at my exhaust between the headers and the o2 sensor and there is a large crack running around the one pipe. Could this be allowing extra oxygen in and messing of the fuel to oxygen ratio of the jeep? Could this make it bog?

Yes an O2 sensor or crack could causes engine problems. I would make an educated guess that it's causing your issue. :thumb:

sevenservices 05-08-2013 03:51 PM

I had the hesitation issue if it was wet. It would bog down pretty bad but not stall... Even splashing through a puddle and it would do that on me. Turns out, it was my coil.

To test it, you can let it idle and dump water all over the coil, if it sputters, your coil prob. has a crack somewhere. Something to test anyways...

Jer91 05-09-2013 07:43 PM

Well I ended up taking the jeep to the shop and had my o2 sensor replaced as well as the wiring to it replaced because the previous owner had rigged something up with cheap wire. I have a crack in the exhaust between the engine and the o2 sensor that also needs to be replaced but I didn't feel like paying 700 dollars for that now so I guess it will just have to wait.

RebelElf 09-22-2013 12:45 AM

Could it be your EGR valve? I have an '89 2.5 TBI and it dies exactly 7 minutes into warm-up, wont stay running on restarts (have to feather the gas pedal to keep it running) and then the engine hesitates on the highway around 50-55 mph (~3000 RPM) until either I give it more gas, or back off the gas at which point sometimes the engine dies while rolling down the highway. The EGR is a very delicate (and supidly designed) upside-down valve with an exposed piston that easily collects oil, moisture and dirt blown into it from the engine fan, shortening its life dramatically. Also, it spends it's life in a purgatory of hot carbon-y exhaust until the computer decides to lets exhaust gas blow through so any oil burning or oil-based fuel additives will cause extra carbon-y build-up plugging it worse from the inside out. Mine has killed 5 EGR's from various combinations of all the aforementioned symptoms. I have now started covering over the EGR diaphragm frame holes with silver metal duct tape and spraying the piston with graphite dry-lube to extend the EGR piston's life. I am also committed to realizing that every bottle of fuel system treatment I've ever used was actually doing a GOOD job at just keeping the EGR alive. Every time mine had an engine sputter, stall, hesitation or other perplexing engine problem, the EGR was typically the cause. I've misdiagnosed its problems as bad fuel pumps, filters, O2's, plugs, caps, rotors, etc., just like you all - often to no avail.

Hanes' manuals have a test for EGR's. Sometimes the engine will run rough or die with the EGR vacuum unplugged but I've never seen that. The smart thing to do is start by tracing the vacuum from engine to EGR solenoid, and solenoid to EGR valve (15" Hg usually). Test the solenoid twice - unplugged to be sure it actually blocks engine vacuum to the EGR, and plugged in (energized) to allow vacuum. I believe the computer controls the voltage in order to open vacuum... otherwise, the solenoid's natural position is closed - which makes sense if you think about it because you don't want exhaust in the intake until you truly need it. I've also come to discover that a handheld vacuum pump also isn't strong enough by itself to move the EGR diaphragm alone because the EGR also operates off exhaust backpressures. Therefore, any exhaust systems modified for less backpressures may not be opening the EGR and could be plugging up the EGR or harming the engine by allowing fuel pre-detonation (pinging - which the EGR was designed to prevent, despite every delusion of clean emissions grandeur). I performed this test, then removed the EGR and checked the diaphragm/piston travel - it was sticky and rough. Sometimes cleaning out the EGR diaphragm and piston rod is possible but its pieces are all built together as one and make a good cleaning or any repair virtually impossible. It really comes down to a choice between doing a half ass job twice, or doing it right the first time - one cheap reprieve, or the expensive fix - as these units range from $80 universals to $120 OEM's. I have replaced my EGR and once again all the stalling and hesitation are gone.

Hope this helps.

RebelElf 10-02-2013 06:39 PM

Still dies/stalls while warming up/decelerating...
I was wrong in my above post - the engine must have still been warm enough to hide the stalling/dying during warm up. It still dies until it warms up, irrespective of outdoor weather, and dies on highway deceleration and coming to stops. However, once the thermostat seems to have cycled open-closed a few times, the dying/stalling disappears. Additionally, the new EGR HAS fixed the highway hesitation and there is no longer the flat spot around 55 MPH (3000 RMP) - that is a definite fix.

Any ideas?
My intuition says that this dying until the engine gets hot is a sensor that relates to engine temp - maybe the Coolant Temp Sensor (CTS) or Manifold Air Temp sensor (MAT)?


RebelElf 10-07-2013 05:47 PM

*** Update - can't edit my above posts on this forum so relegated to replying to myself, sorry ***

I figured out the stalling/dying problem on warm up. Again, the dying/stalling was with a cold engine, and not at normal operating temperatures. And as previously mentioned, the new EGR fixed the flat spot hesitation around 55 MPH (3000 RMP). So now, intuitively, the dying/stalling issue acted like a computer glitch directly based on engine warm-up and thermostat cycle time. The engine didn't sputter, diesel, or otherwise choke down before dying - it literally just shut off in an instant like a computer or electrical cut off. So taking a big fat guess, I replaced the Coolant Temperature Sensor.


No more dying at exactly 7 minutes into warm-up. No more multiple restarts at stop signs or lights before the engine reaches operating temperature. No more dying on deceleration before the engine gets hot. No more feathering the gas to keep it running until it was done with it's temper tantrum. Two test drives with a new CTS under identical conditions have confirmed full success.

Good luck future forum-ers. I hope this helps keep you from Just Emptying Every Pocket to figure out ridiculous Jeep problems because many answers become few and far between the older these dumb elephants get. While the shop manuals have the statistics and details they have no experience. Yet even these experienced forums can't always pinpoint the myriads of interrelated issues that Jeeps are plagued with.

Jeep On,

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