New to square bodys, truck won't stay running

[Badluck]

Hey everyone,

As stated in the title, I am new to chevy pick ups and I love my 87 square body, until my bad luck gets to it! It's a v20 4x4 with a good ole chevy 350 tbi in it. I went wheeling (*never again*) and I was going over a hole that had ice over it. Well needless to say, the front end broke through and went straight down into the mud. When the truck hit the mud the truck stalled. Ever since, the truck will not stay running longer than 20 or so seconds.

To start it I need to:
1. Turn the key on and let the pump prime for a few seconds.
2. Crank motor over. At this time, the service engine light comes on.
3. Once service engine light illuminates, step on the pedal just to give her some gas and she fires up.

It runs great just as it is supposed to for about 20 seconds. Fuel comes out of the injectors just fine, then, they just stop firing fuel into the throttle body.

I have replaced the oil pressure switch.
I have switched the a/c and fuel pump relay plugs.
I don't see any loose connections but I can't seem to find the ground wire for the pump.

I would like to test the fuel psi but these trucks don't have a Schrader valve. Any ideas on how to test this or what else it could be? Any help will be greatly appreciated!

Thanks!!

 

[junkyardog05]

You shouldn't have to give it any throttle to start. Have you checked you line pressure? I would also check that small piece of rubber between the fuel pump and sending unit to make sure that it doesn't have a split in it. That would make it lose pressure to you injectors. Also if you don't give it throttle will it start? You test the fuel pressure where you filter goes into the line at.

 

[Badluck]

If I don't give it gas the truck will stumble but will not start. I have to go buy a pressure gauge tomorrow and figure out how to rig the gauge to the line.

 

[junkyardog05]

Have you checked your intake temperature sensor? Unplug it and see if your truck will start. Whenever that sensor goes bad it gives the ecu a default reading of forty degrees below zero. Sometimes the wire to the sensor will break wear it makes the bend to plug into the sensor. The sensor is located right behind the water neck on the intake.

 

[89Suburban]

When it get's cold here (below freezing) I have to push the pedal too or else it won't start (only on cold start). If I leave the pedal alone it will start on the 2nd try. I am wondering if my butterfly idle adjustment is off because of worn bushings?

 

[junkyardog05]

When it get's cold here (below freezing) I have to push the pedal too or else it won't start (only on cold start). If I leave the pedal alone it will start on the 2nd try. I am wondering if my butterfly idle adjustment is off because of worn bushings?

Its a possibility or IAC (Idle Air Control) valve could be dirty and need a good cleaning.

 

[chengny]

Some info on fuel system testing and operation. From the 1987 GMC_Light_Duty_Truck_Fuel_and_Emissions_Including_Driveability Manual.

Link to full book (262 pages) good info - download and save:

http://depositfiles.com/files/rblr611g2

Is it possible (assuming everything has had time to dry out) that when you crashed through the ice, you might have pinched a fuel line?

 

[Badluck]

Unplugged that sensor and the truck wouldn't even run haha. I'll check the fuel lines again but I didn't see any pinches. Tonight I put 12v directly to the pump so it stayed running. I start the truck and the truck stalled, with the pump still running. I'm thinking it could be an ignition control module?

Also replaced the fuel pump relay today, checked all fuses, getting fuel to the throttle body fine.

 

[chengny]

Seems you are confident that there is fuel available (and it is of sufficient pressure & flow) at the TB inlet. Next step is to see if its a fuel flow problem within the TBI system.

It happens like clockwork right?

Start the engine with the air cleaner off - and have a can of ether in your hand, or nearby.

When it reaches it's 20 second limit and starts to die, hit it with the ether. Shoot it straight down the throat.

If it continues to run, you have a fuel supply issue.

If it dies on schedule - even with a steady supply of ether being pumped down the throat - it is not due to a lack of fuel.

You can then focus your diagnostic efforts on the ignition system (spark).

 

[junkyardog05]

Ok temperature sensor is working correctly then. If the trucks cranks then you have fire, however are you getting enough fire and is it hot enough. You can tell by the color of the spark, I think its supposed to be blue but im not 100 percent on that.

 

[chengny]

It seems to run perfectly for 20 seconds (during that time the arc across the electrodes of a plug would presumably be the classic bright neon blue with an audible snap) and then it dies.

I don't mean to be a wiseguy, but how would he check the condition of the arc after the engine has conked out?

I was going to suggest that if the ether did not keep it running he should next use a inductive timing light to determine if the plugs stop firing just prior to shut down or do they kind of die out as the engine stops.

The timing light would not need to be aimed anywhere, just used as an indication of whether or not the plugs were being supplied from the coil right up till the stall point.

 

[junkyardog05]

After it dies from running the 20 seconds will it start right back up again?

 

[chengny]

After it dies from running the 20 seconds will it start right back up again?

Excellent question, somehow I had gotten the idea that he could restart immediately. Well, almost immediately - at least after waiting just long enough to do this procedure:

To start it I need to:
1. Turn the key on and let the pump prime for a few seconds.
2. Crank motor over. At this time, the service engine light comes on.
3. Once service engine light illuminates, step on the pedal just to give her some gas and she fires up.

Regardless, he's got to do that ether test as the first step in the diagnostic procedure.

If he finds that he can keep it running on ether, the problem is fuel related.

If he cannot run it on ether, the entire fuel system (including all related sensors, switches, hoses and wiring) can be considered to be okay. There are exceptions, but at that point it would be a safe assumption that the issue lies elsewhere.

Next on the diagnostic check list for a non-start would be lack of ignition (i.e. spark). Pulling a plug (and noting the condition of the arc across the electrodes) while cranking the engine would be an easy check of the entire ignition system as the culprit.

But remember, this not a non-start condition.

The engine is stalling only after a normal crank and start up (his start up procedure as noted above is probably SOP). After firing up, there is a brief period of what appears to be normal operation followed by a sudden stall after about 20 seconds.

So, since it is virtually impossible to provide the engine with an external source of ignition (as is the case with using ether as an external fuel supply), things get more involved:

1. Does the engine stall due to a sudden loss of spark as directed by the ECM?

2. Or, does the ECM continue to call for spark, but the spark just cuts out normally (while the engine stalls from some other cause) and begins to slow down?

It is not an exact science by any means, but this is where the use of an inductive timing light comes in.

If the strobe from the timing light is watched carefully as the engine stalls, it might be possible to see if the spark stays strong to the bitter end or does the spark shut off like a switch and then the engine sputters to a stop.

 

[junkyardog05]

It almost sounds like a security issue or an auto shutdown relay, this truck shouldnt have either. The ether test would be my next step.

 

[Badluck]

Found the issue guys! Before I say what it was, I want to thank you all for your ideas and help!

A friend came over and wanted to help me get the trucking running again and well this friend isn't the brightest but he has worked as a mechanic before. I told him to go under the dash and check all the fuses, so without hesitation, he dove right in while I was under the hood inspecting. From in the cab I hear, "they're all good!" Well a couple days later, I wake up in frustration because I couldn't figure the problem out! I go outside and before looking any further under the hood, I said to myself, "let ME check the fuses. Dad always said, if you want it done right, you got to do it yourself." I climbed under there and saw 2 ECM fuses. Pulled the first, it was good. Pulled the second, and it was fried. Ran to walmart, and $3.00 later I had my Chevy running again.

I feel like a moron.