Finding a possible vacuum leak

[krigermoose]

I have a 1986 GMC C1500 with a carburated 350. It drove nicely of the past few days until today. I started it cold and immediately backed it out into the driveway, then shut it off. Later, when I came back to start it again, it started fine, then immediately died. If I give it modest throttle, or light pumps, it will keep running, but occasionally backfires and dies anyway. I can hear a hissing sound coming from the engine bay, especially when I accelerate, so it seems likely that it's a vacuum leak. The choke light also turned on. How would I go about finding the leak? Unless that's not the issue?

[Rapid Roy]

Welcome.

With a cold engine, start it then spray carb cleaner around base of carb and all of the vacuum connections. Engine will rev where there is a leak. Do not recommend to do on a hot engine for possible fire from cleaner.

[krigermoose]

Hey, thanks for the welcome and for responding.

After a few visual inspections (since the engine is still hot so I didn't test yet) I noticed the PCV valve is super loose and covered in oil. Does that cause a vacuum leak? Because I'm definitely getting a new one tomorrow and grommet to see what happens.

[Rapid Roy]

Could possibly be an issue. May need to also replace hose to pcv. They get hard over time and not seal well.

[Irish_Alley]

as long as the pcv has a good hose connection it wont cause a vacuum leak.

[krigermoose]

I'm starting to think it's a fuel issue. Someone recommended that I clear out the fuel pump by pulling off the line to the carb and attacking a line that goes to a clear bottle so I can see the sediment being cleared out, hopefully that will do something as well.

I also may have mistaken the hissing noise for the carb pulling in air, as I hear it the loudest when I accelerate.

I'm honestly trying everything, hopefully something works.

[VileZambonie]

as long as the pcv has a good hose connection it wont cause a vacuum leak.

A PCV valve stuck in the full flow position is like a vacuum leak.

[Irish_Alley]

yeah should of added as long as its "free moving"

[krigermoose]

Update: I installed a new filter, disconnected the HEI battery connection, disconnected fuel line to the carb and connected it to a clear bottle and let the engine turn over for five 15 second intervals. Now, when it starts it gradually revvs up for about 4 seconds then loudly pops smoke out the carb and dies. Any ideas?

[VileZambonie]

Sounds like fuel starvation from what you describe. What was the result of your fuel volume test? Will the engine continue to run on carburetor cleaner?

[krigermoose]

The pulses of fuel looked full, after cranking for 15 seconds I filled up a 1 liter bottle. The truck won’t run on carb cleaner either. The carb just shot flames.

Yesterday I pulled off the distributor cap and noticed the ignition coil had some burns, so I’m going to replace that, then possibly the pickup coil as well.

[VileZambonie]

Probably more cost effective to buy a loaded distributor

[krigermoose]

Probably more cost effective to buy a loaded distributor

I ended up buying a reman distributor. It did not fix the problem.

So far I’ve added an ‘84 corvette fuel filter, and installed a new distributor with new cap, rotor, and coil.

I’m running out of ideas here. It’s definitely no longer an electrical issue though... unless the spark plugs are bad or something but they’re new NGK plugs


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[bd]

Collect a sample of fuel in a clear container.  Is the fuel sample crystal clear and not layered?  Verify I+ at the distributor BAT terminal.  Find TDCC on #1 and remove the distributor cap to verify that the rotor is pointing to the #1 distributor tower.  Recheck your firing order.  Verify a strong blue spark at one of the plugs.

[krigermoose]

UPDATE: The carburetor gasket was the issue. I pulled it off to rebuild it and the gasket was severely blown. Found the vacuum leak...