96' Jimmy-Haunted? Fuel Delivery Problem!

Ok, this is my first post so I'll try to be as quick and as descriptive as possible. I own a 1996 Jimmy SLS 4x4 4 Door. I have been having a very frustratating problem on and off for 3 years now that I can't seem to solve for the life of me. The following is a list of things I have recently replaced myself.

-Fuel sending unit/Tank (I had this done at the shop hope they did it right)
-Fuel Filter
-Air Filter
-Distributor cap/rotor
-Plugs/wires
-Recently had all sensors and electrical wiring checked over. Everything passed.
-Intake manifold gasket set/w/orings
-Broken injector poppet
-Battery
The problem I am having is a fuel delivery based one, I am certain.IT doesn't matter if I'm doing excessive idling, stop and go heavy traffic or bustin down the highway, the engine performance will suddenly start decreasing (rough idle jumping from 500 to 750 rpm contantly, sputtering off the line, rough shifts) until eventually it will just stall,(engine dies,powersteering dies,ect) and won't restart at all. It has stalled off the line, and while I was cruising down the highway at about 85mph once. In any situation, it will not fire up again.....the fuel pump even sounded alot lower in tone compared to the crisp breif whirrr it usually produces. Unless you physically pour some kind of combustable into the throttle body, down into the plenum, it won't fire. Cranks,sparks,no start. Pour gas into it Then it will fire up everytime, but won't stay running for long maybe 3-4 seconds. After sitting for awile (long enough for it to fully cool off, 5-6 hours) It will start fine, run great for weeks (even lays rubber when you mash on it) in cold weather, this has never happened. Always on a warm heavy driving day.

It was leaking coolant, (visibly,externally,and into the combustion chamber and out through the exhaust) due to a blown intake manfold gasket, which I replaced with a higher quality fel-pro gasket.

So I know that my Cat and o2's are probably in bad shape or worse. (i'm assuming the engine is fine because on most days it has excellent power and runs smooth. My question is will a bad leak such as this totally screw my EgR/exhaust system causing my engine to stall and cause a no start condition or is this problem more serious??


-note, the amount of carbon/gunk I found everywhere in the intake manifold and egr passages was sickening.so I had the manifold dipped, and re-machined, due to excessive pitting from the coolant leak on the mating surfaces.

I must fix this, I have been left stranded for the last time!

any help would be greatly appreciated.! THANKS for your time.

When my '94 Astro van acted like this- it was a wire at the connector to the injector spider that was nearly broken in two- held on by only a thread- Once I fashioned new connector wires- all was fine-- it's tough, because there is no way to bench test the connector- I had to just eliminate all other possibilities and happen across the loose wire connection at the crimp. My van acted like your truck for over 6 mths-- came home on a flatbed several times- until it died for good the last time.....

what a fast reply. !Yes I did closly inspect/clean that whole spider assembly and all of its wires, had the pressure regulator vacuum tested too. all was good.

It sounds like your leaking fuel. What is your fuel pressure? You should have 60 to 65 psi with key on and your pressure should stay above 55 psi with key off for the at lease 3 minutes.

It sounds like your leaking fuel. What is your fuel pressure? You should have 60 to 65 psi with key on and your pressure should stay above 55 psi with key off for the at lease 3 minutes.

I checked, the fuel pressure is normal, but I have not tested it while the truck was in a no start state.

A friend suggested that if my cat was blocked it would cause my engine to run hotter than usual, and that excess heat could cause a high resistance on my ignition module, spider injector pins, and other key components, causing them to fail at any given time.

I am havin a full stainless Gibson exhaust with a new High flow Cat, and having the o2 sensors tested. So I'll see if it happens again.


Also... does anyone know anything about this nut kit for the spider?? is it worth having??

For your year, a nut kit is not required. They were more for the "95" and below motors. You need to also look at the leak down on your fuel pressure. Post back your findings. Good Luck.

For your year, a nut kit is not required. They were more for the "95" and below motors. You need to also look at the leak down on your fuel pressure. Post back your findings. Good Luck.
Will do, thanks for the info man. Ill put up some pics of her soon. Btw nice rides u got! Is that a full susp lift or just body on that Blazer?? looks nice!

Will do, thanks for the info man. Ill put up some pics of her soon. Btw nice rides u got! Is that a full susp lift or just body on that Blazer?? looks nice!
Thanks. Its a full Susp. Your welcome. Good Luck. Post your findings.

The "Nut Kit" is used on ALL "W" and "X" engines. It's the fuel pressure and return line assembly that goes to the injector and from fuel pressure regulator. They are about 7" in length and the upper intake manifold, (plenum) must be removed to service them.

The line to the injector can be checked for leaks during a fuel pressure and leakdown test, without removing the plenum.

Ok after it dies again on me, I finally took it to the shop and here is what I am being told.

-No start condition I only have around 35psi pressure, so they told me the fuel pump was bad and had to be replaced.

-I just replaced the whole sending unit, and had a new release valve assembly put in her about 10 months ago.

-The shop assured me that the fuel pump was bad. So The next day I had them go out and try to start it, it started right up and ran fine.

If my fuel pump was bad, why the hell does it restart and run fine for weeks/months after it been fully cooled down??

Now they are saying they have to drop the tank to properly test everything. I can't afford to replace the sending unit once per year... is there anything that is known to cause fuel pressure to drop intermittently?? Im assuming that it would not have started up and ran/ drove fine with the psi in the 30's since it needs to be about double that normally.

Is there anything I should be telling them to look for in particular? Please any help is appreciated greatly!

Has the injector unit been replaced, or checked for leaking? My 99 would start fine first try in the morning, and for the rest of the day have trouble starting. There were small leaks at the injector body. Replaced the spider and all is well.

Has the injector unit been replaced, or checked for leaking? My 99 would start fine first try in the morning, and for the rest of the day have trouble starting. There were small leaks at the injector body. Replaced the spider and all is well.

Well when I had it all apart to change the intake gaskets, Before I put the upper plenum back on I turned the key and cycled the pump about 5 times, while paying close attention to the spider assembly, I didnt see any visable leaks coming from the spider at all, it actaully looked like it had been replaced in the last 5 years or so, also vac tested the pressure regulator and it was holding vac fine.

Just because the pressure is 35psi does NOT necessarily mean the pump is bad. Several things can cause insufficient fuel pressure. A poor electrical connection anywhere in the circuit such as the fuse, ECM, fuel pump relay, fuel pump module, fuel pump motor, relay ground or pump ground. It could be a faulty or weak pump, or a fuel leak either internal or external. When it acts up, you need to have a fuel pressure tester connected to determine if the pump is running and what the pressure is, if it is holding pressure, and if the pump is capable of building sufficient pressure to satisfy the fuel pressure regulator.

This is what I thought. They jumped straight to the conclusion that the pump was bad, without even considering most of the things you mentioned. It was acting up at the shop, then the next morning it starts/drives fine again, to me this sounds more like an electrical as opposed to a mechanical thing.

So I'm going to try driving it home tonight when there is less traffic around and it's colder outside. Wish me luck it's only about 10 miles.

Really tho, at the cost of an oem GM fuel sending unit you would expect it should last more than 20000 ish miles. wtf. I always have over half a tank, and I have changed fuel filter twice in last year alone.....

Thanks again -OM- This problem will be getting fixed because of this forum, im sure of it. Ill post back when I know for sure. Have a gooder everyone.

OK, here is an update. I finally gave up and took my truck to the dealer since it was only 6 blocks away from the shop it was at. The truck ran poorly, and it died right as I pulled into the bay. Im thinking what good luck, it died in front of them and now it won't restart.

They call me up today saying that when they got to it this morning, it started right up, ran fine all day, and they could not get anything to malfunction. His exact words were "It is frustrating me because it wouldn't start for me yesterday at all, but when I wen't out in the morning it ran like a champ. We tested the fuel pump it's good,the truck was purring like a kitten all day, you can come pick it up or leave it with us until something happens" which always seems to be at the wrong time. So what the hell?

I did notice it was colder out today, I live in Calgary so it's not uncommon for winter one day and summer the next. On the days it has died it was always been warm outside. Could this be temperature related somehow? overheating P.C.M? Ignition module? I only drove it 6 blocks yesterday and it died, although it was sitting in the hot sun all day so I can't see it overheating that fast. Im Thinking that I should tell them to do a proper leakdown test??

The dealer, of all people, should know that to diagnose an intermittent problem, you need to have the test equipment connected when the problem is happening. All the testing and checking in the world won't find a problem that's not there.

so what did you finally find out? iM having same problem with my 96 astro!

its been there for 3 days now and they can't get it to malfunction, like Om said they can't find a problem if it isn't happening. The guy said instead of racking up some more shop hours, I should just pick it up, so I am going to right now. I have been reading the article OM wrote regarding fuel pressure on my cmfi, I have a few idea's I'll post back.

its been there for 3 days now and they can't get it to malfunction, like Om said they can't find a problem if it isn't happening. The guy said instead of racking up some more shop hours, I should just pick it up, so I am going to right now. I have been reading the article OM wrote regarding fuel pressure on my cmfi, I have a few idea's I'll post back.

Dude, your problem sounds almost EXACTLY like mine. I was driving down the interstate....bam! engine cuts off at 70mph! I pulled off, looked under the hood. No leaks, smells or damage that I could see. It sat for 15 min and started up, running rough. I got 1/4 down the road and it stalled again! I looked under the car and see fluid near the transmission. But it looks like it's coming from about the tranny somewhere.

Temp was normal and everything so I know it didn't overheat. Fuel pump maybe? The fluid didn't have a gas smell. I'm at a loss here.

Jay:crying:

Well I thought I had it solved again, fuel pressure was holding strong at dealership. I figured out how to make it die now... it has to be heated up and driven, then shut off and quickly started (about 2-3 mins off) just enough time for the heat to build up. It will still only malfuntion on a hot day. I changed the deteriorating fusible link/pos battery cable out, she drove fine for a couple weeks, then as soon as I started punsihing it on a hot day, it quit.

It is obvious to me now that it is a hot thing, What could possibly be overheating on the top end of my engine and causing my fuel pressure to drop into the 30's, and make the fuel pump to start sounding different?

It could be the fuel pressure regulator, any one of the lines in the plenum, or the check valve in the fuel pump. The pump needs to be isolated from the system when the problem occurs, the results will determine if the problem is the fuel pump or in the plenum.

Old Master, I was reading your article on fuel pressure and I was about to get started when I noticed something.

My fuel system is "CSEFI" which doesn't seem to appear in your article. I checked my VIN, the 8th character is a W. So would the info still apply even though it is not CMFI or CSFI? Or maybe CSEFI and CSFI are one and the same?

Thanks again.!

CSFI, (Central Sequential Fuel Injection) and CSEFI, (Central Sequential Electronic Fuel Injection) are the same thing, just a fancier name.

Hey, apologies for the extremely late update.... and resurrecting this thread but I have recently returned to this forum and wanted to close this thread out properly.:smokin:

Anyway, I ended up replacing the fuel sending unit (again) at the GMC dealership... around 20 months ago, along with a ton of other upgrades I added myself which I don't really want to list in this thread. Pics and info are coming in the appropriate thread.

Long story short, I did the repair at a quality, (and equally expensive) repair shop and it seemed to make a huge difference. I have put roughly 50000KM on the Shimmy since the repair(s), and the problem has not returned. It will now start and run fine in even the worst of conditions. :)

I now realize how important and critical proper fuel pressure is with these engines, and how an unqualified or overconfident mechanic could easily do an improper repair, resulting in fuel delivery failure at any given time, for any number of reasons.

Mods*/Admin* feel free to close off this thread if you wish. Thanks for all the help guys!

Hey, apologies for the extremely late update.... and resurrecting this thread but I have recently returned to this forum and wanted to close this thread out properly.:smokin:

Anyway, I ended up replacing the fuel sending unit (again) at the GMC dealership... around 20 months ago, along with a ton of other upgrades I added myself which I don't really want to list in this thread. Pics and info are coming in the appropriate thread.

Long story short, I did the repair at a quality, (and equally expensive) repair shop and it seemed to make a huge difference. I have put roughly 50000KM on the Shimmy since the repair(s), and the problem has not returned. It will now start and run fine in even the worst of conditions. :)

I now realize how important and critical proper fuel pressure is with these engines, and how an unqualified or overconfident mechanic could easily do an improper repair, resulting in fuel delivery failure at any given time, for any number of reasons.

Mods*/Admin* feel free to close off this thread if you wish. Thanks for all the help guys!

This is exactly why if you suspect your fuel pump you should purchase a fuel gauge and tape it to your windshield, then when your truck dies you know exactly what your pump is doing when it dies. In this case most likely it loses pressure.