Won't Start...need help please.

[Racer-G]

Hi

I have a 1987 Chevy Scottsdale C30 short axle dually with a 350 tbi, 400 auto, with duel tanks. recent fuel pumps and in cab switch. New coil, plugs and wires all done with in the last 6 months. New map sensor about a year ago. It has been running great .

I drove 30 miles to the mall, was doing some Christmas shopping, looking for a parking spot when all of a sudden the motor just quit. I rolled into a spot and restarted the truck, no problem. Could not see anything wrong under the hood. Did some shopping came back out, started right up. Drove for 15 minutes quit again saw the engine light. Stopped put it in park, switched the tank started right up. I pulled into a parking space. Seeing the engine light reminded me that I needed to top off the oil, it was a quart low. Got back in and it started right up. Drove down the highway another 5 minutes quit again. Coasted to the off ramp put it in park. But this time it would not start. tried a couple of more times and still nothing. The ECMB2 fuse was blown. Replaced it, started up but was not wanting to accelerate. When I gave it gas it wanted to die and it was running rough, so I just let it idle while I got under the hood. It was getting gas just running rough. About a minute later it quit again. Tried to start it, nothing. Yup ECMB2 fuse blown again. Damn no 10 amp fuses left, lets try a 15 amp. Popped it in and started up still running ruff. It didn't take long and I started to smell that smell I just hate the most, burning plastic. So I shut her down immediately and the fuse was blown.

I got her towed to were I was staying.

The next day I got a box of 10 amp fuses. Having been stranded off the highway I never could hear the fuel pumps work. I popped in the 10 amp and it would not start. Both left and right pumps sounded normal like they always do when I start it up. The fuse was not blowing. I poured some gas in the throttle body, hit the key and it started up and ran until it was out. The fuse was not blown. Tested both fuel pump relays with another new one I had with me, so its not them.
I am at my mothers house about 700 miles from home and I don't have an electric tester.

It is at this point where my knowledge is limited and I could sure use some help, ok a lot of help. I'd like to get back home some day.

Thanks in advance for all your help. Hope you all had a great Christmas. I know I did considering.
Racer-G

 

[89Suburban]

Yikes! I am not in front of my schematics right now or I would help. Hopefully some others here will chime in a out that certain fuse and its circuit. When I get home tonight I will check back in.

 

[chengny]

It appears that you are not the first Chevy owner to experience this issue:

If you enter "ecmb fuse keeps blowing" in the search field - Google lights up like Christmas tree.

I quickly scanned several threads (on maybe 5 different Chevy/GMC forums) - so maybe 20 total cases. This is what I found:

I know you are in a hurry to get started on this repair, so I won't bore you with the details.

There does seem to be a common area where the short occurs. In those threads where a root cause was identified (and repaired), the wiring fault was usually found to be in a group of wires behind the back of the block/intake manifold.

This group exits the main cross-over harness (about at the trucks's centerline). It travels up the rise of the transmission bellhousing, continues up the back of the engine block and then the group splits off to the various components that are located on the top of the engine.

Before you take the following results too seriously; understand that this was not a scientific sampling - I just skimmed to the bottom of lots of threads and tried to keep track of the most common causes.

The wires most commonly found to be damaged and shorted to ground were:

1. The sensing leads to the oil pressure switch (BTW - the power supply to the fuel pump may be fed through the oil pressure switch)

2. Various leads that connect to the distributor.

Most people reported (just as you did), that their first attempt at a diagnosis was to change the FPR's. I don't recall any cases where that solved the problem.

Ultimately, repeated blowing of the ECM-B fuse was nearly always found to be mechanically damaged wiring in the area between the firewall and the top/rear of the engine block.

 

[Racer-G]

Thanks Paw Paw and Jerry
I did forget to mention that I did check the wires by the firewall. I also looked at a bunch of posts in this and other forums before I posted this. Most were fuel pump or lack of spark related and some posts just ended without giving their results. I did double check the firewall today after I read your posts just in case I missed something. My wiring at the distributer and oil pressure switch have that plastic shielding and are not that close to the firewall and I don't see any chafing. I pulled out the glove box so I could see the ECM unit. All the wires looked or felt in tact.

Jerry, I didn't find as may posts as you did on the subject. But I will keep trying. I think that because the 1987 model is a little more unique (last year of body style, first year of tbi ect...) it might be more difficult to dignose. Did this year truck have a plug in to do diagnostic checks? If so where is it located? Also if what I think is correct, does the oil pressure switch its self have something to do with startup, and could it be the switch? And is the oil pressure switch actually a switch or do they just call the oil pressure sending unit that?

 

[89Suburban]

Thanks Paw Paw and Jerry
I did forget to mention that I did check the wires by the firewall. I also looked at a bunch of posts in this and other forums before I posted this. Most were fuel pump or lack of spark related and some posts just ended without giving their results. I did double check the firewall today after I read your posts just in case I missed something. My wiring at the distributer and oil pressure switch have that plastic shielding and are not that close to the firewall and I don't see any chafing. I pulled out the glove box so I could see the ECM unit. All the wires looked or felt in tact.

Jerry, I didn't find as may posts as you did on the subject. But I will keep trying. I think that because the 1987 model is a little more unique (last year of body style, first year of tbi ect...) it might be more difficult to dignose. Did this year truck have a plug in to do diagnostic checks? If so where is it located? Also if what I think is correct, does the oil pressure switch its self have something to do with startup, and could it be the switch? And is the oil pressure switch actually a switch or do they just call the oil pressure sending unit that?

Yes there is an ALDL plug under the dash near the parking brake that you can use a jumper wire to check for codes. and yes there is an oil pressure switch involved in the ECM/TBI start up process.

Hopefully these diagrams will help you trace the short:

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[Racer-G]

Paw Paw your awesome.

I won't be able to get to work on the truck until Wednesday evening, but man you made this easy for me and I thank you.

HAPPY NEW YEARS in advance to EVERYONE :

 

[chengny]

An view of the ECMB (essentially the fuel pump) circuit with description of operation:

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The same dwg - edited to show only the sections of the circuit that apply to your truck - and could blow the ECMB fuse:

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A brief explanation of why the oil pressure switch is included in the circuit:

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It is a common misconception that it's there to kill power to the FPR/fuel pump if the engine stops running. Truth is, it is actually there to provide an alternate source of power to the fuel pump should the FPR fail.

 

[Racer-G]

I jerry rigged a test light last night with a turn signal bulb and some wire, (no offence Jerry). It seems I have no power at the ECMB fuse.

I'll need to go to the auto store and get some better test equipment . Anyone have a suggestion on what to get. I can turn a wrench as well as any one, but when it comes to electrical I'm a little lost and slow.

All your help is very much appreciated. I can't thank you all enough. Best wishes for a happy, healthy and prosperous New Year.
Racer-G