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-   -   91 Cherokee rough idle, stumbling acceleration, fuel pressure drops fast (http://www.jeepforum.com/forum/f11/91-cherokee-rough-idle-stumbling-acceleration-fuel-pressure-drops-fast-1361643/)

noir 04-26-2012 02:59 PM

91 Cherokee rough idle, stumbling acceleration, fuel pressure drops fast
 
Hi guys, I a little hesitant to post because I'm a huge lurker but not tech-savy enough to answer other's questions. However I'm at the point now that I may need to bring my XJ in and have a mechanic look at it, and I figured I'd make one last-ditch effort by posting and maybe save myself some $$$. I don't have a lot of tools.

The Jeep
1991 Cherokee HO 4.0 L6

The symptoms:
It began maybe a month ago, I noticed that upon starting it would run rough for a few seconds then smooth out and be fine. Then a few weeks ago I started it and it ran rough and didn't smooth out. Since then I've had rough idling, intermittent rough running, some stumbling and loss of power on acceleration. The sometimes it will go from a rough idle to being smoother on acceleration, then back to rough when idling. It ALWAYS stumbles/has loss of power on full pedal. I've noticed that coming off a stop, I can hit the gas a few times harder when accelerating and let off the pedal completely, and then press it gently back to accelerate and it will be smooth. Weird.

I've also noticed that it has been running significantly hotter than normal.

New parts:
During the time I've spent lurking these forums, I've singled out some common suspect parts and replaced them (all brand new parts).
Plugs
Wires
Cap
Rotor
TPS
IAC
MAP
O2 sensor
Cat-back dynomax exhaust (thought I might have a back-pressure problem)
Fuel filter and hoses

Fuel Pressure test:
Today I borrowed a fuel pressure tester and found something interesting. On idle, the fuel pressure at the rail is ~32 psi, when vac disconnected, the pressure is ~40 psi. This is the correct range according to my manual. However from an idle to key off, the pressure immediately falls to 10psi (in about 3 seconds), after approx 10-15 seconds the pressure reads zero.

In the past 6(?) years that I've had the jeep, I've noticed that it always takes quite a few cranks to start.


Jeep quirks/relevant info
Electrical aux fan is on a cab switch bypass and works normally.
Exhaust manifold has the notorious crack in "Y" just before the downpipe.
Radiator is less than a year old.
No check engine light shown, but ran the codes and it spat out "35" which makes sense considering my fan bypass.
Just pulled the throttle body off and cleaned it real good, no changes in symptoms.

What now?
Here are several possibilities that I can think of:
Bad injector(s)
Weak fuel pump
Timing chain slipped

I want to rule out the fuel pump because it maintains pressure on idle, and I'm not sure if it would run intermittently "normal" if it was a timing chain issue. And I really don't think I have the technical skill to test the injectors.


Any ideas?

Reptile 04-27-2012 12:47 PM

You did the right thing taking the fuel pressure. Yes a fuel pump can work intermittently, trust me I know. The drop in pressure after u turn the ignition off is symptom of a bad check valve; also explains the cranks when you start it. You might want to start replacing parts going from cheaper to more expensive: relay to fuel pump (just swap it), fuel filter (you already did) then fuel pump. Also try bypassing the ballast resistor.

Injectors are pretty easy but i doubt that's what it is. They're more likely going to give you cyl misfires.

Map (check wiring) and O2 sensors also suspect. Also clean the IAC and check your ground and battery connections.

noir 04-27-2012 04:03 PM

Thanks for the reply. What relay are you talking about? The little fuel pump relay that's in the fuze box? And what's a ballast resister?

Yesterday I took the throttle body off and cleaned it out real good with hot soapy water, throttle body cleaner, and a toothbrush. It looks much better but didn't fix the problem.

Someone told me to check the compression on the cylinders but I'll have to borrow a tester for that. Maybe I'll have time next weekend. In the meantime in gonna do a hd fan clutch swap that I've been putting off.

Reptile 04-27-2012 11:45 PM

Yes, the fuel pump relay is in the fuse box under the hood (it isn't little though). Ballast resistor regulates the fuel pump; it's under the hood, driver side, run a wire across to bypass it.

It's way to early to do a compression test. Your issue is intermittent, so you're more likely looking at an electrical issue. You don't want to rule out anything though.

Did you check all your grounds/battery connections? This is very important. You want tight connections and no signs of corrosion.
When you say it's running hotter, how hot are you talking about? Does it coincide with your intermittent issue?
What brand the new O2 sensor?

tjwalker 04-28-2012 06:01 AM

Quote:

Originally Posted by noir (Post 13474123)

I've also noticed that it has been running significantly hotter than normal.

I'd make it a priority to address this first. Running too hot is never good. We need details about what you mean by hotter than normal. What is the coolant temp gauge reading? Does it run hoter at idle only? On the highway? Both?

First thing I do for a warm reading is to verify it by pointing an infrared point and shoot thermometer at the thermostat housing of a hot idling engine.

BTW, your fuel pump is most likely working correctly, (pressure is good but volume has not been tested) but the extended crank times on cold start are being caused by a loss of fuel pressure most likely because of a faulty check valve on the fuel pump as Reptile mentioned.

I would test the map sensor for your driveability issue. You'll need a meter and a manual. And perform a close inspection of the vacuum hose that leads to that sensor, paying special attention to the ends which often crack with age. Be sure that vacuum line is pulling good vacuum.

noir 04-28-2012 09:42 AM

Quote:

Originally Posted by Reptile (Post 13481974)
Yes, the fuel pump relay is in the fuse box under the hood (it isn't little though). Ballast resistor regulates the fuel pump; it's under the hood, driver side, run a wire across to bypass it.

It's way to early to do a compression test. Your issue is intermittent, so you're more likely looking at an electrical issue. You don't want to rule out anything though.

Did you check all your grounds/battery connections? This is very important. You want tight connections and no signs of corrosion.
When you say it's running hotter, how hot are you talking about? Does it coincide with your intermittent issue?
What brand the new O2 sensor?

I'll get a new relay and try shorting that ballast. I'm pretty sure the O2 sensor is a Bosch, I'll try to find my receipt though. As far as the ground connections, I'll look at my manual and try to find the ground points (I know there's a small one just off the neg battery terminal to the side of the frame, also on on the drivers side near the fuel rail to the firewall).

The heat issue may be due to a faulty gauge, and does not coincide with the issues. I have the symptoms across the board, hot or cold.


Quote:

Originally Posted by tjwalker (Post 13482344)
I'd make it a priority to address this first. Running too hot is never good. We need details about what you mean by hotter than normal. What is the coolant temp gauge reading? Does it run hoter at idle only? On the highway? Both?

First thing I do for a warm reading is to verify it by pointing an infrared point and shoot thermometer at the thermostat housing of a hot idling engine.

BTW, your fuel pump is most likely working correctly, (pressure is good but volume has not been tested) but the extended crank times on cold start are being caused by a loss of fuel pressure most likely because of a faulty check valve on the fuel pump as Reptile mentioned.

I would test the map sensor for your driveability issue. You'll need a meter and a manual. And perform a close inspection of the vacuum hose that leads to that sensor, paying special attention to the ends which often crack with age. Be sure that vacuum line is pulling good vacuum.

The heat issue may be due to a faulty sensor, I've replaced the thermostat housing less than a year ago but just used the old sensor. I'm picking up a new sensor today. I don't have an IR gun.

If the coolant gauge is correct then it seems to run fine at idle and climb when I get on the freeway or especially when I am climbing hills at city speeds (live in Seattle). I've noticed that I never get a "roar" from my mech fan so I'm going to do a clutch swap because I think it's bad.

I have my electrical fan on a switch so I usually just leave that sucker on all the time and it seems to keep the temp in control.

The map sensor is brand new (replaced less than a week ago) and I inspected all my vac lines and they all seem to be in good physical condition, but I didn't check to see if they are pulling vacuum -that's a good idea.

Reptile 04-28-2012 11:09 AM

I agree with TJ, I would be concerned about the temperature. Funny, I just found my long way out of an overheating issue after rad swap. I ended up replacing the thermostat (twice), water pump, temp sensor and gauge.

The sensor at the thermostat housing is connected to the ECU, not the gauge. This is the sensor that's supposed to kick the aux fan on, you can easily find a replacement part. The sensor that goes to the gauge on your model is at the back of the engine, it's hidden. To find the part, good luck. The right sensors is hard to find because they are calibrated to match the gauge. Your best bet is the junk yard. What I ended up doing was getting a new gauge kit at O'reilly for about 25 bucks. It came with the sensor.

The other ground points you want to check are the ones connected to the engine. One is at the dipstick, the other is connected to a stud at the back of the engine (ground strap), actually near where the temp sender unit is at.

noir 04-28-2012 02:40 PM

Ok, I'm done working on it today; I have about 8 hours of homework to catch up on. Tomorrow I'll clean those two ground points. I swapped the fan clutch out for a hd one and that sensor on the front. I guess I didn't need it, ah crap $25 wasted cause my fan is on a bypass switch. Oh well.

I found the real temp sensor, it's small and on the drivers side towards the back of the block. I'll have to grab one from a junkyard sometime.

Also, my radiator and thermostat and thermostat housing are all less than a year old and my coolant looks clear, no cloudiness so I don't think I have a radiator block. And the top hose gets hot so the thermostat should be working.

I'll post back tomorrow with results from the ground cleaning.

armmata 04-28-2012 09:42 PM

for the ground you should have 3 body, chassy, and motor if your fuel pressure is droping any amount you eather have a leaking injector or yes the fuel pump is going out. about a year ago i was having the same issues and it turned out after going through the same things u have it was the fuel pump that was my fix if you really don't need the new parts why buy them you can find fuel pumps on ebay cheep

noir 04-30-2012 11:57 PM

Sorry for the delay. I'm having to put this project on hold for a few days to focus on class and help out family. I had scheduled to bring it into a shop on Friday but I might cancel that and replace my injectors and fuel pump myself this weekend and then see if that resolves the issue. The dilemma is do I spend the money replacing what eventually needs to be replaced anyway or take it in and have the exact problem pointed out (for a stepp price). Bleh.

Thanks for the help so far.

Reptile 05-01-2012 12:08 AM

Quote:

Originally Posted by noir (Post 13496938)
Sorry for the delay. I'm having to put this project on hold for a few days to focus on class and help out family. I had scheduled to bring it into a shop on Friday but I might cancel that and replace my injectors and fuel pump myself this weekend and then see if that resolves the issue. The dilemma is do I spend the money replacing what eventually needs to be replaced anyway or take it in and have the exact problem pointed out (for a stepp price). Bleh.

Thanks for the help so far.

Depends on how much you feel comfortable dealing with pure evil (mechanics). You can always ask to pay for a diagnosis. Tell 'em you're a student you can only afford the diagnosis and want to know how much you need to save to fix it. Then take the diagnosis and fix it yourself. Make sure you define the scope of diagnosis, since you have two apparent, yet separate issues. Don't ever suggest that you have any money, wear dirty jeans/shirt, etc. Whatever you do, make sure you get a quote before they even touch it.

armmata 05-01-2012 12:41 AM

Your best bet would be fuel pump first if you can handle a hammer and punch and are willing to crawl under the rear of the jeep i'm sure u could handle the job if u need im available to call and. I could walk u through changing the pump

noir 05-08-2012 09:40 PM

Ok I'm back. Thank you guys very much for the info and help. I ended up researching and finding a praised offroad and repair shop that specializes in Jeeps.

They found the electrical and fuel systems to be fine. Their eventual diagnosis was that my intake valve on the #4 cylinder is sticking. They're sending me a repair quote but I already know what that will say haha.

So, anyone recommend a good place to order a rebuilt head from?

Reptile 05-09-2012 12:28 AM

Quote:

Originally Posted by noir (Post 13539284)
Their eventual diagnosis was that my intake valve on the #4 cylinder is sticking.

Based on what was their diagnosis?

noir 05-09-2012 07:58 AM

Quote:

Originally Posted by Reptile (Post 13539766)
Based on what was their diagnosis?

I'm not sure exactly. When he sends me the quote today I'll give them a call and ask more questions about it. This was a line from the email:

"Yours has proven to be quite frustrating and difficult to diagnose. *You had covered all of the easy stuff, but we still had to double check some of it. *Bottom line is we have proven the problem does NOT lie in the ignition system, or fuel delivery system. *We have isolated that the majority (if not all) of the problem lies in the #4 cylinder. *What I believe we are dealing with is a sticking intake valve on # 4 cylinder."


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