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1986 GMC Jimmy Fuel Delivery Issues

Lunatic

Lunatic

1/2 ton status
Joined
Jul 27, 2008
Posts
439
Reaction score
4
Location
Ft Hood, Texas
Hello everyone,



My 86 GMC Jimmy is having a fuel problem. Running a 383 and a Rochester carb, mechanical fuel pump.

From zero, I can start the truck with no problem. It will die within about 20-25 minutes, idling in the driveway - No fuel - It will not start again while at operating temp. Walk away for an hour, and after the initial re-priming, it fires right up. Rinse and repeat.

I have checked numerous possibilities - And have yet to find the actual problem. For simplicity, I'll just put everything in a list form and go from there.
  • Carb pump sucking air - Carb rebuilt, SMI CA
  • Mech fuel pump faulty - New pump installed
  • Mech fuel pump arm to rod - Checked
  • Mech fuel pump rod condition - Smooth, no bends
  • Fuel tank supply tube screen clogged - New sending unit installed
  • Debris in tank (original 86) - Drained, cleaned out tank
  • Rubber hoses sucking air - All hoses for fuel supply, return replaced
  • Clogged steel lines - Disconnected, blown out with pressurized air

The only thing I haven't changed is ventilation lines for the charcoal canister to the tank, from the frame joint forward. All rubber from the steel line frame joint back is new. Only the supply and return is new forward of the steel line frame joint.

My head tells me something is sucking air, and that it is heat related. I just cant wrap my mind around it. A vapor lock is suspect - But, after 3 years (383 install), how and why "all of a sudden".

I'm prepared to install an electric fuel pump, regulator, gauge etc to get around this but I'd prefer to understand and fix the actual problem.


Any ideas ? Your help and suggestions are appreciated !



Troy B
Ft Hood, TX
 
I know you have checked this, but I have to bring it up. As soon as the truck quits, and will not restart, have you looked into the carb throat while you pumped the gas?
If it is a fuel problem, there should be no gas squirting into the intake.
If there is, then its probably the module in the dist. getting hot and dying.

And, of course, if you have a propane torch, you can turn on the gas without lighting it, and cranking the truck with the torch blowing into the carb.
The truck will not run great or fast, but it should start on the propane, which would eliminate an ignition problem.
 
Hey Fordum !

So, yeah - You are correct in believing that I have checked that. There is absolutely no fuel. I layed over the engine with my nose down the throat while it was being turned over by second guy. I have even cranked it with no supply line between the carb and fuel pump - Nothing at all

The biggest issue I have is that a mechanical fuel pump, regardless of electrics, should be pumping via the rod, by the cam lobe, when the engine is being turned over. But no fuel, until it is cooled down some. Something is screwy.

Something else I failed to mention in the initial post - The steel supply line from the mech pump to the carb is in like new condition and a clear passage.

Right now I think I'm going to disconnect my charcoal, so I can get it off my mind - Even tho my mind finds no relation to the issue other than the common fuel tank.

I'm open to any other ideas you may think up Fordum.


Thanks for the response !


Troy B
Ft Hood, TX
 
Good to hear from you again.

Glad you mentioned removing fuel line from pump to carb. That eliminated three or so suggestions I would have made. I agree about the charcoal, not likely, but something to try.
The only way I could see it causing problems would be if it were stopped up completely and letting a vacuum form on the tank. But if you loosened the gas cap, it would bypass that problem.

If I remember, once the fuel system is primed from the tank to the pump, cracking the fitting at the pump input should let fuel siphon out of the tank.
But, I own Fords, so I may be wrong.
Your best bet at this point, might be to fab a piece of line to hook to the input of the pump and run it to a separate fuel container.

If you have any kind of vacuum gadget, like the little hand vacuum tester they sell in parts houses, you could try taking the fuel input line off the pump and see if you can suck any fuel out.

If it runs out or you can get any, then its going to be the pump or the actuator. I suppose the little "cup" on the end of the cam could be working loose when hot somehow and spinning on the end of the cam.
I doubt it would have lasted very long that way though.

The only other idea I have, is to loosen the intake line and take the output line off the pump. if you put your finger over the output line, it should pump some air pressure when the engine is spun over.

I don't think I have ever seen a mechanical fuel pump fail and then go back to work. The only time I remember seeing a fuel starvation problem that would fix its self and then happen again, was a rag or piece of plastic wrap floating in the gas tank.
It would get sucked over the intake and stay there until the suction went away, and drift off and the truck would run great for a while.

To tell the truth, I would hold off on the electric pump for a while. Unless its a problem with the new mechanical fuel pump, an electric one might just mask the problem until it got bad.
 
I'd second the electric pump avoidance. If there was a reason to run one, it would be due to certain requirements...the OEM mech. pumps are very reliable and flat out work. Electrics introduce a lot of additional potential failure points.

At least you know the problem isn't in the carb. As Fordum mentioned, loosening the fuel cap would eliminate the EVAP canister as a culprit. If your tank "hisses" when you remove the cap, something is not breathing correctly, which is the EVAP system.
 
There's no intention of running an electric pump.

Loosened my cap last night, pulled the charcoal can line, and started the engine.

Grabbed a sander and repaired the shroud on my Windstar setup. Engine got to temp, cycled the fan, and eventually starved out again.

25-30 minutes.

I think what I'll do next is verify the steel lines are actually clear-clear. Blowing pressurized air thru a line is one thing - But sending a string thru and pulling a clearing / cleaning plug back thru will verify 100%.

From there, I'll focus on the pump, and pump to cam relations.

Thanks again for the ideas.


Troy B
Ft Hood TX
 
Lunatic said:
Hello everyone,



My 86 GMC Jimmy is having a fuel problem. Running a 383 and a Rochester carb, mechanical fuel pump.

From zero, I can start the truck with no problem. It will die within about 20-25 minutes, idling in the driveway - No fuel - It will not start again while at operating temp. Walk away for an hour, and after the initial re-priming, it fires right up. Rinse and repeat.

I have checked numerous possibilities - And have yet to find the actual problem. For simplicity, I'll just put everything in a list form and go from there.
  • Carb pump sucking air - Carb rebuilt, SMI CA
  • Mech fuel pump faulty - New pump installed
  • Mech fuel pump arm to rod - Checked
  • Mech fuel pump rod condition - Smooth, no bends
  • Fuel tank supply tube screen clogged - New sending unit installed
  • Debris in tank (original 86) - Drained, cleaned out tank
  • Rubber hoses sucking air - All hoses for fuel supply, return replaced
  • Clogged steel lines - Disconnected, blown out with pressurized air

The only thing I haven't changed is ventilation lines for the charcoal canister to the tank, from the frame joint forward. All rubber from the steel line frame joint back is new. Only the supply and return is new forward of the steel line frame joint.

My head tells me something is sucking air, and that it is heat related. I just cant wrap my mind around it. A vapor lock is suspect - But, after 3 years (383 install), how and why "all of a sudden".

I'm prepared to install an electric fuel pump, regulator, gauge etc to get around this but I'd prefer to understand and fix the actual problem.


Any ideas ? Your help and suggestions are appreciated !



Troy B
Ft Hood, TX
Click to expand...
You didn't mention anything about the fuel filter
 
So I had another chance to return to this issue. Below is what I found.

Fuel filter - Failed to mention, sorry - It is new, inline at carb housing.
Coolant low - Pump mounts to engine block - Engine block has cooling passages is where I came up with this. Filled now to correct level.

With system completely assembled, fuel cap and all - Ran truck for the initial 15 minutes before it sputtered and died. Applied water hose over pump for a minute and it started right back up after a few coughs. Left water hose head on pump housing and the truck ran for an hour before I called it off and quit for the day.

Vapor lock at the pump. I'm still not tracking on the "why now" part, but that will come maybe. I will mention that I'll be getting some fresh fuel, being as this fuel is from last summer. I've been reading up on the ethanol and separation etc. Its one more item (fuel) I can remove from the equation.

Everything else I read online says electric fuel pump - I'm still not there yet.


Thoughts ?


Troy B
Ft Hood TX
 
Is your fuel return line connected and clear? It should be allowing any fuel that isn't being used by the carb to return to the tank so that should allow your pump to move a constant supply of fuel and not vapor lock.
 
There may be a 1986 rubber fuel hose somewhere between the tank and fuel pump that has now separated inside thanks to ethanol,and it closes off after awhile from the suction of the fuel pump..

My 70's K5 Blazer had similar symptoms ,it would die out going up steep hills at highway speeds,or after merging onto the highway if you had to floor it and hold it there till you reached the speed limit..just as you almost got to a complete stop,the engine would "catch" and fire back up again,it rarely stalled out completely--it just refused to rev up most of the time..if you babied it,it wouldn't die out at all..around town it was fine..

I replaced the fuel filter,blew the steel line out with compressed air ,and it seemed un-restricted,also replaced the fuel pump,and since the carb had a dying accelerator pump I rebuilt it also--to no avail..

I was told by an older mechanic to make sure the steel line had no pinholes on the top side,especially near any bends,that could let air in,but not "leak" fuel--that was OK..
But during that inspection, I noticed a foot long rubber hose near the rear wheel,that joined 2 sections of the steel line,and it looked original,it had the oem squeeze clamps,and the lines looked original too--I cut it off and replaced it with new hose,and that cured it..

I sliced the old hose lengthwise with a razor knife and the inside was all gooey like bubble gum..evidently it choked off the fuel supply just enough to starve the fuel pump under heavy loads or on takeoffs..took me a long time to figure that one out--and I almost got into several accidents because of the truck wanting to bog down when I needed to "boot it" and merge into traffic..I had to stay off the highways..
 
Small update:

All lines have been cleared using wire and pipe cleaner (sized for tubes). All rubber hoses for line connections are new.

The entire fuel system, at some point, has been in my hand - Literally.

Tonight I dropped my dual 3" exhaust to eliminate 2 cats and 2 cherry bombs - Of which the right side was terribly close to the frame rail and fuel lines. Im going to replace this 3" system now with 2.5" regardless.

Started the truck, straight headers - 25 minute run time before stall. Didn't bother with the water hose on the fuel pump this time - I was simply removing the cats from the equation.


I will continue to look into this issue as time permits - Any additional suggestions are always welcome !


Troy B
Ft Hood TX
 
I get the same 'running out of fuel' problem if I let my 86 K5 with an electric pump sit and idle for 10 minutes or so. I built the 350 stroker and just went out and got an electric pump b/o previous mechanical pump failures for some ungodly reason.

But, yeah....my electric pump still has to suck fuel from the tank, and that lowers the boiling point of gasoline quite a bit and I get vapor lock between the pump and the tank - just two feet away!
Some day I'll pack the pump and suction line with insulation - someday. As I get older, I get lazy-er.

Until then I just make sure to keep moving for some ram air effect under the truck and all is well.
 
It is implied, but want to verify...the return line is verified clear as well?

You've got a new sender, so the return fitting shouldn't be an issue.

Really shouldn't have a vapor lock problem with a return system, and like you said, why now if it worked fine prior?
 
dyeager, sorry I missed your comment. To answer your question definitively - Yes, the return line is completely clear. Initially with pressurized air and finally with a welding line and dish washing pipe cleaner pulled through.

For my final straw, I am going to run the fuel return into a bucket directly under the mechanical pump, effectively doing 2 things:
(1) Elimination of old fuel - The fuel from the return side will not be going back in the tank, ever. This fuel has sat too long and is in questionable condition.
(2) Removing the carried heat of the return line fuel to the tank, then eventually back to supply, will also provide an answer of some kind.

I will provide feedback before Friday night. Any further suggestions are always welcome.


Troy B
Ft Hood, TX
 
I assume you removed the stock fuel filter that is in the carb behind the fuel inlet fitting,and installed an external universal one ?..

If not,the paper element filter in the carb might by plugging up or the rubber check valve thats in it might be faulty..
 
Results.............

The engine ran for 92 minutes (according to timer) today before it died. 3X longer than the normal 25 minutes.

The difference this time though - It ran out of fuel (!) which is a good thing also.

So, with the return line running directly from the pump to a bucket on the ground right below - It HAS to be the return line. Even though I pulled the pipe cleaner through. At this point, I don't care that it doesn't make sense.

The run time alone is enough for me to pull the trigger on a new LMC return line ($85) and while at it I'll replace the supply ($85) as well. Before installation however, I will be insulating with rubber hose "just because" along the entire frame rail.


Opinions / comments ?


Thanks for all the input throughout this drawn out problem - The brotherhood rocks as always !



Troy B
Ft Hood, TX
 
I know this is older, but I would probably just have used rubber line to test.

Note that I suspect using rubber as an insulator is a bad idea. The return line may act as a cooler, insulating it will not allow any heat to be shed, which will serve to increase the tank fuel temperature, which will then result in an increase in fuel inlet temp at the engine.

Obviously you'd need to check the fuel temp coming out the return side once the engine is warmed up and compare to ambient temp to see if my assumption is correct. And of course, the longer the vehicle runs, the higher the fuel temp is likely to get.

When my fuel pump got weak, the reduction in volume through the fuel lines caused the fuel at the engine to absorb a lot more heat, resulting in it running like garbage. It was obvious enough that you could tell simply by touching the fuel rails. Under normal operation the rails were cool to the touch, with less volume, they were hot enough to be uncomfortable.
 
So I chose to verify the return line theory. I ran a length of hose from the pump return to the tank return.

Back to 25 minute run times, with fresh fuel.

As dyeager pointed out, rubber is an insulator - but insulation works both ways.

Currently I am waiting on 2 coolers to arrive. One double pass heat sink cylinder for the supply, which I will put outside the frame where the 2 steel lines are coupled with rubber hose. And one 4-6 row, 10x10x¾" conventional cooler for return, which will be mounted in front of the radiator, coming from the pump return and then back to the tank return.

And being that I have NEVER needed or seen my secondary fan actuated on the Windstar - I'm going to do a tiny rewire on it so both fans become primary, drawing even more fresh air to the engine.

I will report back with results when I know the outcome.

Thanks again for the continued input, brotherhood.

Troy B
FT Hood TX
 
One idea would be to use rubber line to insulate FROM the heat source, and allow the rest of the line to be open to ambient. It's funny when you really start digging into stuff like this that is generally considered "simple", how complex it can really be.
 
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