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Problem with my tow vehicle no one else can help


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21 replies to this topic

#1 Ionlyhave3suubs

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Posted 12 May 2011 - 06:18 PM

I am having problems with my 96 GMC Suburban and can't seem to get help elsewhere. It has a bad miss under load, it seems intermittant, and I cant track it down. I have done the basic tune up things, Plugs, wires, cap, rotor, air filter, fuel filter, vacuum leak check, inspection of wiring harnesses to sensors. No CEL, no codes when scanned. The engine is a 454. here is a link the forum where I was trying to get help.

http://www.truckforu...html#post201331

as of my last posting I had 57 reads and no replies.

If anyone has ideas let me know. This is my truck I use to tow a camper from VA to Myrtle Beach every summer and to pull my car trailer for new purchases. I also drive it about once a week around town.

thanks in advance!

#2 MilesFox

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Posted 12 May 2011 - 06:27 PM

slack timing chain?

#3 Ionlyhave3suubs

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Posted 12 May 2011 - 06:32 PM

Timing chain, I haven't checked that yet.

#4 3eyedwagon

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Posted 12 May 2011 - 06:49 PM

o2 sensor can create a stumble, and not throw codes. If you are handy with a multi-meter you can check what it's reading, and see if it needs replacing. If not, they don't cost a ton to replace, and are a good maintenance item that will save you some gas $ anyways.

On the big blocks the 95-96 era was transitional. The earlier styled distributor had a hall-effect switch that could cause odd problems at times. Check to see what distributor you have. If your hall effect switch is bad, you may want to end up plopping a whole new distributor in, as changing that switch can be kind of a PITA.

You may want to consider a fuel pressure gauge. A tired pump can create pressure for idle, but, be too weak to keep up when the engine needs more fuel. A fuel pressure gauge being monitored while revving the engine will show you if there's a problem or not.

Other than that, you are on the right track. It will just take a little time to troubleshoot. This generation of upright throttle bodied injection is super simple. You can figure it out. Continue to check sensors with a multi-meter and you will find it.

PS: The Holley, and HEI would be a stupid, stupid move. Less efficiency, and more complicated in the long run.

EDIT: Just read about the o2 sensor so never mind.

Edited by 3eyedwagon, 12 May 2011 - 06:53 PM.


#5 davebugs

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Posted 12 May 2011 - 07:43 PM

Back in the day we had a few 454 Dually's at an RV place where I worked.

Had trouble with the trucks running well when bringing back trailers from Elkhart and such.

Ended up beingt brand new cats. We tried using the redneck method - a cold chisel through the cat and mufflers. Worked like a charm. Then we fixed it proper.

A day later we got a load of LEER truck caps. Guy used the phone to call home and complain about how crappy the truck ran when hot. Guess what we did? Loud but worked. I assume they "fixed" that truck when it got back. Brand new Chevy trucks.

I'm fichting an intermittent miss in my 97 Astro van AWD 4.3 Vortex. Changed 3 fuel filters, all the normal tuneup stuff. Gonna put a new fuel pump in next week.

I only notice when towing, and never a CEL. DOn't forget the crappy gas they force us to use. Local mechanics have been seeing more burnt valves in cars so far this year it's amazing. I just got back form my local independent power equipment dealer and he was telling me how bad it's still getting for his stuff.

So I'll be following along with curiosity to see what ends up fixing your problem. It may help me with my issues. And yes - I've checked my cats.

#6 3eyedwagon

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Posted 12 May 2011 - 08:26 PM

If it's a Cat you'll know in short order. When they go the honeycomb usually drops apart pretty quickly creating a very large clog. I could see how it could be confused for a fuel shortage, but, a quick trip to the exhaust tip while someone else revs it will tell you whether or not that's the case. They will usually blow a bit of crap out the tail pipe, sometimes even big chunks of sooty fuel soaked garbage. Even if it's not forcing much out the pipe you'll be able to hear/see its' labored breathing pretty easily at the exhaust tip.

#7 Ionlyhave3suubs

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Posted 12 May 2011 - 10:26 PM

I'll most likely start working on it again on Sat. Tomorrow I have to work 8am to 11pm so I won't get to it tomorrow. If I get it resolved I'll post what it was.

This engine is not the throttle body fuel injection type, it has the multi-injector vortec system. My "93 I used to have had the TBI, now that one REALLY sucked the gas! 10mpg on a good day!

BTW, I did replace the cam sensor (it's in the distributor), didn't help.

Have also tried buying gas from different stations, wasn't bad gas.

#8 3eyedwagon

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Posted 13 May 2011 - 12:16 AM

This engine is not the throttle body fuel injection type, it has the multi-injector vortec system. My "93 I used to have had the TBI, now that one REALLY sucked the gas! 10mpg on a good day!.


Let me know the 8th digit of the VIN, that will tell me what injection system we are working with. They used 3 injection systems in that era (TBI, MPFI, and CPFI) even on the same years, and it can get confusing. If it really does have an injector for each cylinder on a fuel rail it is MPFI. Either way, they all have multiple injectors, and they are all Vortec systems. Vortec refers to many (depending on model) post 1993 heads, not the fuel delivery system.

The MPFI systems have a few more sensors, but, still aren't overly complicated. It's really sounding like a fuel issue to me. If it is MPFI the fuel rail probably has a threaded nipple ready to have a fuel pressure gauge installed for these exact diagnostic purposes.

#9 VaporTrail

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Posted 13 May 2011 - 10:14 PM

I had a CPFI system on a smaller engine, and it was doing the same thing. people had me changing the EGR valve, and the whole injection system practically, but it ended up being a fuel pressure leak in one of the relief valves under the truck....

#10 Ionlyhave3suubs

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Posted 14 May 2011 - 10:23 AM

The 8th character in the vin is a "J". Where can I get an appropriate fuel gauge to test the pressure? This is a multi point fuel injection system. Is there a secondary filter on this thing I need to check? I changed the one that is mounted near the frame about mid-ship.

#11 Ionlyhave3suubs

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Posted 14 May 2011 - 02:19 PM

Allright, I picked up a pressure gauge at Advance Auto, It reads about 50psi at idle in park or neutral. Haynes manual says the proper range is 56-62 psi. Apparently the proper term for this fuel injection is Sequential Fuel Injection (SFI). And it does have 8 injectors hidden under the top half of the 2 piece intake manifold.

Haynes says to slowly pinch the flexible return hose to see if pressure increases to normal range. If so, to replace the pressure regulator. It's raining hard right now and I don't have any help to watch the gauge while I pinch the line.

I noticed when in neutral or park the pressure seemed to hover right at 50psi. In drive, it fluctuated up and down about 5psi intermittently. In drive, giving it throttle holding it stopped with brakes, the pressure would climb into the 60psi range where it should be at about 2000 rpm.

#12 Ionlyhave3suubs

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Posted 16 May 2011 - 07:46 PM

I found that the vacuum hose going to the fuel pressure regulator was deteriorated and cracked, replaced the regulator and hose, problem persists. According to Haynes manual if you pinch the return hose and the pressure goes up, replace the pressure regulator. I did, it did, I did, still not fixed. Looking at fuel pump next. Fuel pressure still at only around 50 psi. Why doesn't Subaru make something big enough to pull big trailers?

#13 3eyedwagon

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Posted 16 May 2011 - 09:13 PM

5psi is plenty enough to starve out a FI Big Block under load. In other words; 5psi would be noticeable, and I'd bet it's the fuel pump.

Anytime you test a fuel pump on a fuel injected system, and it isn't meeting factory specs (even 1 or 2 psi low) that is probably your problem. Can't guarantee it for you, but, I know that'd certainly be my next step.

As for your complaint of it not being a Subaru... do you really think that changing an in tank pump is easier from one vehicle to another? :rolleyes::lol: In tank is in tank. They all seem like a PITA until you've done a few. Just pop the bed up, and yank it out the top. That's the best way I've found. Just be careful with all the fuel lines....:eek: don't want to kink one. That'll increase the duration, and cost of the job about 300%.

#14 Ionlyhave3suubs

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Posted 17 May 2011 - 06:12 PM

Fuel pump is definitely my next destination. My comment about it not being a Subaru was more about how long it has taken to track down the problem, mainly due to how difficult lots of the components are to get to. I think I am on week 5 of trying to resolve this driveability issue, at one point I gave up on it and took it to a mechanic, and they couldn't track it down after having it a week. So I dug back into it, still working at it, have a good idea now what to do next, but as of yet, still not fixed.

Dropping the tank looks like pretty much the only way to get to the pump. The bed/cab is all one piece, like an overgrown station wagon.

Then there is the sawsall to the floor of the bed approachzzz

#15 3eyedwagon

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Posted 18 May 2011 - 12:10 AM

Fuel pump is definitely my next destination. My comment about it not being a Subaru was more about how long it has taken to track down the problem, mainly due to how difficult lots of the components are to get to. I think I am on week 5 of trying to resolve this driveability issue, at one point I gave up on it and took it to a mechanic, and they couldn't track it down after having it a week. So I dug back into it, still working at it, have a good idea now what to do next, but as of yet, still not fixed.

Dropping the tank looks like pretty much the only way to get to the pump. The bed/cab is all one piece, like an overgrown station wagon.

Then there is the sawsall to the floor of the bed approachzzz


Sorry, totally forgot about it being a Suburban. I pretty much automatically thought of a pickup, because that's what I've done these on. I've taken one out through the bottom, fortunately GM put some long winded bolts on the strap assembly. Drain the tank best you can and they aren't too bad to wrestle.

In all honesty, if the mechanic couldn't track down the problem on this fuel system, I'd not return to that mechanic. A fuel pressure gauge being screwed on to the factory nipple isn't exactly advanced diagnostics. This should be shop 101 to a professional.

#16 Ionlyhave3suubs

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Posted 18 May 2011 - 10:02 PM

Fuel pump replaced, no change other than a lighter wallet, dirty clothes, and a nice gasoline odor to myself. Fuel pressure gauge still reading about 49-50 psi at idle. I notice as the engine "stumbles" (miss or something like a miss) the fuel pressure goes up slightly until it recovers from the "stumble" (a matter of less than 2 seconds).

Oh well, go fish. Next on the list, check for timing chain slop.

#17 3eyedwagon

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Posted 19 May 2011 - 01:31 PM

Fuel pump replaced, no change other than a lighter wallet, dirty clothes, and a nice gasoline odor to myself. Fuel pressure gauge still reading about 49-50 psi at idle. I notice as the engine "stumbles" (miss or something like a miss) the fuel pressure goes up slightly until it recovers from the "stumble" (a matter of less than 2 seconds).

Oh well, go fish. Next on the list, check for timing chain slop.


If a stumble is recovered by an increase in fuel pressure I'd still say it is fuel related. Hard to say without having it parked in my garage, but, from what you describe that would still be my bet. If I get down to my buddies shop in the next couple days I'll dig through his manuals, but, it still sounds to me like you are describing a fuel delivery problem.

Sorry to hear you're having such tough luck.:mad:

#18 Ionlyhave3suubs

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Posted 19 May 2011 - 09:09 PM

If a stumble is recovered by an increase in fuel pressure I'd still say it is fuel related. Hard to say without having it parked in my garage, but, from what you describe that would still be my bet. If I get down to my buddies shop in the next couple days I'll dig through his manuals, but, it still sounds to me like you are describing a fuel delivery problem.

Sorry to hear you're having such tough luck.:mad:


Maybe I was unclear, as the stumble happens, that's when the pressure goes up. As soon as the stumble goes away, fuel pressure goes back down to 50psi, running smooth. It looks as if the stumble causes the brief pressure increase.
Possibly an interruption in signal to the injectors causing pressure to build for a moment while injector(s) not working?

#19 Ricearu

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Posted 25 May 2011 - 01:21 PM

im gonna go with drop in engine vacuum as it misfires, which causes the FPR to momentarily raise pressure, remember, less vacuum equals more load. im gonna say timing chain slack, or maybe just a sticking intake valve... just a shot.

#20 Fairtax4me

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Posted 06 June 2011 - 12:19 PM

im gonna go with drop in engine vacuum as it misfires, which causes the FPR to momentarily raise pressure, remember, less vacuum equals more load. im gonna say timing chain slack, or maybe just a sticking intake valve... just a shot.


X2.

I was on the fuel pump bandwagon. But I guess that ones out the window.

I know chevy engines like to ping, kinda wonder if this could be a faulty knock sensor.
I didn't see anything mention of the O2 sensor ever being changed.

#21 Ionlyhave3suubs

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Posted 22 June 2011 - 09:41 PM

Finally got it sorted out. Turns out the problem that got me started working on the truck was a cracked distributor cap. As I worked on it, I replaced cap, rotor and plugs at the same time. I still had the problem, just for a different reason. Well after changing the fuel pressure regulator and dropping the enormous fuel tank to replace the pump with a new one, that turned out to be defective, then the next new one was also factory defective, finally got a good one installed and still had the problem, that turned out to be a misfire, from the #8 spark plug being cracked. Not sure if the crack was a factory defect or happened at installation. Apparently the crack was not bad enough to misfire on every ignition cycle for that cylinder, which made it so hard to diagnose.

#22 3eyedwagon

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Posted 22 June 2011 - 10:59 PM

Sounds like you had a run of poorly built parts causing you to chase your tail. I've had similar things happen with bad coils from Echlin. That crap gets frustrating in a hot hurry. Glad you got it fixed. Way to stick in there.:rolleyes::)




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