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Old 06-01-2014, 01:07 AM   #1
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Default Safari van no start

Ok, so I'm at the end of my rope here. So maybe you guys will come up with something I've missed. I've been trying to help a friend fix his GMC van for sometime now. Details follow:

1994 GMC Safari van
4.3L TBI engine
Cranks fine, will not start unless gas pedal is held in.

It has spark. Good enough to light up a spark tester, and make your hand go numb for a few minutes. It has fuel, we can see the spray pattern of the throttle body. Looks good, and fuel pressure is 12 PSI where it should be. The engine passed a compression test, cylinders between 160 and 180 PSI.

The MAP sensor is getting a good 5V reference, and to the best of my ability to test giving good signals. The throttle position sensor was changed by the owner during his own diag, and IAC taken off and cleaned. Tune up was done about a year and a half ago. All wires have been looked at and are undamaged, plugs have been pulled and look fine. New cap & rotor were installed during diagnosis because old distributor was found cracked.


At this point we replaced the computer, because after relaying this to the master tech I work with he said the symptoms seem to correlate with a van he once worked on. Fair enough because I was at a dead end, but that hasn't had any change. We went back to the drawing board.

I pulled the #1 plug and put it at TDC to check timing. The marks and rotor seemed to line up, but with the engine running, and timing advance disabled the spark is firing about 8 degrees late. I figured maybe we botched the first test and the timing chain had to have slipped a tooth. Which brings us to today. I tore into it to replace the chain, and the timing wasn't off a lick. Well, since I was in there I went ahead and replaced the chain, but now I'm stumped.

It has fuel. It has spark. There'* no air intake blockage that I can see. I don't see any cracked vacuum hoses. New plugs, wires, cap, and rotor. Plug wires are set in the right firing order. Engine is timed properly and passes a compression check. Rotor is timed properly. MAP sensor is working, TPS was changed, IAC cleaned and seems operational. New computer, no DTCs were set before replacement.

For all intents and purposes this engine should run, but it seems bound and determined to break the laws of physics and mechanics as I know them. Anyone have a clue what I could be missing here?
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Old 06-01-2014, 05:47 AM   #2
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When it does run, how far open are you holding the throttle open?
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Old 06-01-2014, 08:39 AM   #3
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Somewhere in the neighborhood of 25%.
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Old 06-01-2014, 11:12 AM   #4
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You mention it has fuel. What is the fuel pressure? The fuel pump is the number one reason for a Safari/ S10/ Blazer/ Bravada/ Jimmy not starting. Not sure why it would start with pedal fully depressed, but I would be looking at the fuel pressure and also the remote possibility of fuel pump ground.
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Old 06-01-2014, 03:44 PM   #5
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Well, I thought of that but the van'* fuel pressure is 12 psi. As I understand it, throttle body injection runs the same as old carbureted vehicles in the neighborhood of 9-13 psi. So it has fuel pressure. And a relatively new fuel filter. I pulled it off and was able to blow through it fairly easy, and no fuel lines are kinked or bent.

It really isn't making sense to me.
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Old 06-01-2014, 04:12 PM   #6
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Quote:
Originally Posted by Marshal View Post
Well, I thought of that but the van'* fuel pressure is 12 psi. As I understand it, throttle body injection runs the same as old carbureted vehicles in the neighborhood of 9-13 psi. So it has fuel pressure. And a relatively new fuel filter. I pulled it off and was able to blow through it fairly easy, and no fuel lines are kinked or bent.

It really isn't making sense to me.
I may be having sometimers syndrome but I thought the 4.3L fi engines need fuel pressure between 58-62 psi- and it should hold that pressure.
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Old 06-01-2014, 05:04 PM   #7
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I may be having sometimers syndrome but I thought the 4.3L fi engines need fuel pressure between 58-62 psi- and it should hold that pressure.
The direct injected models with the spider fuel distributor do need about 60 psi to operate. This is a throttle body injected model.
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Old 06-01-2014, 10:05 PM   #8
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If the engine starts and runs fine with a small crack of throttle, then I would bet the IAC is not actuating as its suppose to.

And he is correct, the TBI engines only need a small amount of fuel pressure for the 2 injectors.
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Old 06-01-2014, 10:09 PM   #9
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Update: Got the timing chain buttoned up today. Took the EGR valve off the engine to see if it was stuck open. It was. So while I was cleaning it I tried to start it out of curiosity, and she fired right up. So with a cleaned EGR valve and new chain, it works. Darned if I know which one fixed it but I'm leaning towards EGR because the timing marks lined up perfectly when the old chain was still on the engine.
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Old 06-02-2014, 08:41 AM   #10
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those chains wont even start making it run funny till 300k let alone not let it start. 10-12 psi is good for tbi. EGR is a common issue. i havent heard of one making it not start though, but it makes sense
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