GMC Truck Part VI

[Czech]

Ok you wench heads and armchair fixer diy guys, here's the latest on my truck.

History: GMC Yukon 5.7L V8 gas, 235,000 rounds. Did the fuel return regulator last fall and a fuel pump last year, while I was doing that I did plugs and wires too, ran fine afterward and until the recent problem.
Problem Now: Intermittent starting problems, cranks but does not start, started this symptom about a month or two ago then maybe every two weeks or so I'd have the problem, it will start with a little spray of starting fluid. Runs great when started, good mileage. Last week it got more frequent, say twice a day, time for a code read. Coded out the Throttle Position Sensor, and the typical 'I have no cats' codes. Replaced the TPS that night, bought a fuel filter just for maint reasons, didn't do the filter as I thought the TPS was the problem and the symptoms to me didn't say fuel restriction problems (push the throttle down it and it goes well down the highway). So this morning and tonight, it would not start without the magic spray, once started runs fine, and would restart right away no problem. Had the codes read again tonight, nothing new. Took it home and screwed on the fuel pressure gauge to the rail. Key on pump spun up and shut off, 58 psi. Started it up, running pressure at 52 psi or so. Shut off and key on right after, sitting about 60 psi. Left sit, losing around 1 psi per 10 minutes or so. I tried to clamp off the fuel pump lines (non-oem high pressure fuel lines) to isolate the problem, but not sure if I did as the pressure on the rail still fell and I was using hemostats (hospital guy here, robbed my tackle box) to do so.

My Thoughts: I have a fuel pressure problem related to the pump or the return regulator, not an electrical/ignition problem as it will run once started? I'm thinking the rail pressure upon start up is right on the edge of opening poppets and that once started it runs fine. Not sure why the TPS low voltage coded, but thinking regardless I'm dealing with the pump or the regulator, either of which at the least will screw up my weekend or at the worst will cause me to go in the backyard with a handgun. Kidding about the hand gun somewhat. If the consensus is fuel pump (that I've done 4 times already, drop the tank, blah, blah), anyone had luck with cutting an access hole in the floor of one of these rigs? It's got 235,000 on it, no worries!


Apologies for the long winded post, I wanted to provide the most detail as possible for you guys (and make up for a long time and little posting). Thanks as always,

Bob

 

[2.beans]

check your fuel pressure when it wont start. if you have good psi then the ecm is not firing the injectors. does the truck build oil pressure alright? does your code reader have a data stream so you can see what the other sensors are telling the ecm ?

 

[savageactor7]

GotzTheHotz had a similar problem with a Lumina van.

Under the hood there was a sensor that told the computer fuel pressure was OK when it wasn't. End result was the pump didn't pump when starting but when starting fluid was used vehicle ran OK. A good wrench figure it out and replaced the shorted wire. With the very rare problem it's SOP to replace the fuel pump...but that won't fix it. Good look.

 

[Mmaul]

Had similar problem with my Jeep ended up being that gasket around the fuel pump on the gas tank. it would sit over night and would not start till the second try.

 

[Czech]

Thanks guys. The pump does spin up and shut off at key on, so it is working. As far as the ECM, if that were goofy wouldn't it be all the time, i.e. no start with the magic spray? Oil pressure is good, having the codes read at the parts store, so no data stream. Other than the pressures listed above, I have not had the opportunity to get a pressure when it won't start. It started fine this morning after sitting all night, driving me nuts. I'd rather it just totally fail so I could track it down! I plan to put the new fuel filter on tonight, I don't think that's what it is but I already bought it. Who knows. I swear if it is the fuel pump, I'm taking the dremel tool to the floor of the rig, man I hate that job. And I need gas on my way home tonight to boot, I don't think I'll be filling the 30 gallon tank! Thanks again all.

 

[Czech]

Didn't start again today after work, sprayed it and it started and ran fine. If it was the ECM or electrical wouldn't that throw codes? Nuts here, absolutely nuts.

 

[2.beans]

that style needs injector needs high fuel pressure even to open the injector, electrically it fill fire but mechanically it wont open the popit. cranking the engine can drawing enough amperage to draw the fuel pump down so it cant keep up. i know these are trucks are real finicky to fuel pressure. if your fuel pressure gauge is a good one leave it hooked. that way you can see it when it wont start.

 

[Czech]

Did the fuel filter this eve just for the hell of it, it needed it. Still having problems. After I got the air out of the lines, ran fine again. Went to start it an hour or so later, no go w/o the help of the magic spray. Runs fine after that, but I did notice the fuel pump wasn't spinning up after every key on. Checked the grounds wire wise, too lazy to pull out the volt meter on the pump, looking at the floor under the back seat to see if the demel can make me an access door. Ah well, at this point to me, it's either the reg or the pump, and for some reason I'm thinking pump. Where's Highbeam? Not that I don't appreciate the input thus far, but I have to tell you, that guy is good.....

Wish me luck, I still plan to tow the 17' Alum down to the St. Croix River Friday eve for fishing, I'll pull it back by hand if I have to, wanna go that bad.

Thanks again,

GTH

 

[Czech]

BTW, in my long winded post, I forgot to mention it was a '97 if that matters.

 

[brad068]

gtw, I have found when dealing with fuel pumps to buy genuine GM or Ford or Chrysler. The aftermarket ones are junk even if they are their top of the line!

It may cost twice as much but it will work better than any aftermarket.

 

[2.beans]

defiantly buy oem parts. a gm pump has a 12/12 warranty and they'll change it for you if its faulty. when checking voltage at the fuel pump are you pushing the leads into the connector? is so check to see if the connector spades are bent in at all. ive seen this a few times. also when gm went with this style injection the early modles had a pulsator in the tank just above the pump that would go be and cause extended crank times, no start and tons of drivability problems. the end fix was to remove it and install a piece of high pressure fuel injection hose in its place. if your buying pump assemblies you shouldnt run into this but if your swaping the pump out of the fuel module you may. another thing that kills these high pressure pumps is water, did you drain your filter into a glass jar and let it sit a bit to see if water seperated from the fuel? if you end up swaping out your pump look in the tank to see if you can see water in the bottom. ive seen tanks 1/4 of water and the truck still runs but very poorly.

 

[Czech]

The last pump was after market, good advice for OEM I guess. I just figure I'll drop that kind of money and then the tranny will fall out or the head will fail! I did not see any water in the filter, and I did check the tank (siphon hose) and that looked good. As far as running, it's a champ, goes great, just the starting issue. The filter needed replacing, it had some nasty crap in there, my bad. It's starting today, I had to leave work early to go fishing, as long as I get on the water I don't care where I sleep tonight. Thanks gang for all the advise, keep it coming and I'll keep you posted. I did pull the back seat up, I'm really thinking if it's the pump I may cut an access door!

 

[Highbeam]

It does matter that it is a 97 since prior to 96 they were a TBI system with one or two injectors and you have 8 injectors, and 8 nozzles buried inside the intake manifold of your "vortec" motor. From 96 to 98.5 the 350 was available as you have it, I liked that motor.

I've been following the thread but can't say for sure. The fuel pump in my 98 vortec died and more recently it died on the 2001 of similar design. They were both all or nothing but the 98 gave me one restart with the help of a bang on the bottom of the fuel tank to get it spinning. The fuel pumps on these trucks are noisy when they work and when they fail they are silent. Not many weak pumps.

The other usual suspects on the vortec motors cause poor running and since yours sounds like it runs fine once it starts I don't really know what's wrong. Intermittent problems are always harder to solve. Good luck.

 

[woodsman23]

Why not try a firewall mounted electric fuel pump??. They are like 30 bucks and you just hook up your fuel feed line from the tank and hook up the power and see what happens. I have a electric fuel pump on my plow truck due too the cam one being worn out and have never had any issues with it. It is an older truck 84 blazer (350) I'd try it this would eliminate the tank one and i mounted mine on the passenger side wheel well. Works great

 

[Highbeam]

The OEM fuel system runs at about 60 psi vs the 3-5 psi for that external pump.

 

[2.beans]

It does matter that it is a 97 since prior to 96 they were a TBI system with one or two injectors and you have 8 injectors, and 8 nozzles buried inside the intake manifold of your "vortec" motor. From 96 to 98.5 the 350 was available as you have it, I liked that motor.

I've been following the thread but can't say for sure. The fuel pump in my 98 vortec died and more recently it died on the 2001 of similar design. They were both all or nothing but the 98 gave me one restart with the help of a bang on the bottom of the fuel tank to get it spinning. The fuel pumps on these trucks are noisy when they work and when they fail they are silent. Not many weak pumps.

The other usual suspects on the vortec motors cause poor running and since yours sounds like it runs fine once it starts I don't really know what's wrong. Intermittent problems are always harder to solve. Good luck.

1995 was the last year for tbi 1996 is when they went sequential fuel injection. ive got one in my shop right now

 

[2.beans]

The OEM fuel system runs at about 60 psi vs the 3-5 psi for that external pump.

you can buy external pumps to run any pressure that you want. ive tried this on a hot rod project and the pump failed early. im guessing from to much heat. the fuel in the tank helps cool the pump. also runnig towards 60 psi you dont want that leaking externally.

 

[2.beans]

The OEM fuel system runs at about 60 psi vs the 3-5 psi for that external pump.

sorry was trying to quote woodsman

 

[Czech]

Just a quick update, the truck has been starting dependably since the fuel filter was changed. I have not checked the rail pressure since then, wonder if it was restricted enough to cause the pressure to drop some? Otherwise I'm sure I'll deal with something else in the near future. Thanks again for all the ideas, I do appreciate it!