Seems like a vacuum leak but... it's intermittent. What the?

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ch4d4

ch4d4

10 Year 15 Year
Joined
Sep 21, 2010
Threads
8
Messages
34
Location
Hilton Head, South Carolina
So on my de-smogged 79 fj40 I have a situation. It was running great, then one day I started it up and it exhibited all of the signs of a vacuum leak (would not run unless choke pulled out etc.). I replaced the fuel filter and pcv as well as searching for a leak buy spraying combustibles. I did get a surge when the base of the carb was hit with the spray however, I couldn't narrow it down to an exact spot. I drove (choke out) a day later and the problem went away during the drive and for about a month. Now it's back. What does everyone think..., continue to chase a vacuum leak or look at dirty carb etc.? Is a vacuum leak ever intermittent? I mean I could see it going away once the engine heats up, but not just going away for weeks.
What gives?
 
Either the carb is loose at the base or the carb base gaskets are toast. Also check all vacuum hoses for cracks.
 
As metal heats up, it expands.
It is common for a leaky intake manifold to leak more, or less depending on how hot it is.
Also intake leaks can be intermittent based on what the shred of remaining gasket decides to do that day.

You may not have a leaky intake, but yes, vac leaks can be elusive.
 
It just seems strange that it goes from pretty good (pulling 20lbs at idle, steady needle) to won't stay running even hot without choke out. Ya know? from nothing to significant vac leak, back to nothing.
I'll keep searching :-)
 
I had intended on removing the solenoid to make sure something wasn't gummed up in there. As for the voltage, I have had to repair the ECM years ago. Can-o-worms!

Thanks for all the advice and observations everyone!
 
Intakes sometimes crack where they are mated to the exhaust manifold. I would look there and make sure its not the brake booster as well.
 
picture of a welded intake crack below:
image.webp

Not to say that is what the issue is with your engine... but it happens, and can create an intermittent vac leak as the manifold heats up.
(This pic is a manifold on a FJ60 2F).
 
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weber adapters are GARBAGE and are known to be a source of leaks...if you have a weber, start by placing it in the recycling bin in your alley, finish by installing a rebuilt Aisin or a THC carb...I guess MAF made a one piece adapter that solves the weber 2 piece adapter problem...also- if you have the VCV for evap still in the mix, check it for an internal leak...the Aisin carb do need cleaning/rebuilding every 10-15 years or so...,HTH
 
7 years on a rebuild might be it depending on previous wear/quality of rebuild/ quality of incoming air/ quality of ring seal and PCV function...
 
Pulled the idle solenoid and it's getting voltage and appears to be operating. The gauge with the choke pulled (engine cold) is now about 17lbs and shaky. I'm not seeing a crack in the intake manifold, but it's a pretty tight space to get a visual. I'm gonna keep looking for the leak.
 
Put vac pressure on your brake booster and see if it holds, I am going thru this again after putting in a rebuilt like 7 months ago. Bad luck I guess.
 
was this ever solved? i have same issue
 
rghouse said:
was this ever solved? i have same issue
Click to expand...

Not sure if this will help, but I think I finally solved the stumble I only had when my engine was cold (FWIW my ‘79 is is not desmogged).

I had a vacuum leak at the HAI thermo valve under the air filter housing. The stumble was so bad I had to drive it half-choked for the first couple minutes after a cold start then it was fine. Makes perfect sense now that I understand what the HAI system does (thanks MUD)!
 
ya mine is a major leak that evens out when the block gets near the 3rd tick on the temp guage, guessing above 180. I have to block the air intake 80-90% or keep choke out and rpms above 1200 or so to keep it idle.

I am going to crank down the carb bolts and see if that helps, i just rebuilt it and I dont think I tightened them super good. I’m not sure what else to check on the manifold, but with it better when hot obviously it is an expansion of some components that make a seal.

i tested the ABV and the ASV (i think) trying to go through manual steps but it was very difficult. Air did blow out properly. it is strange to see what happens when one or two airlines are disconnected.

spraying starter fluid in spots has had mixed results because it is tough to keep running and then it seals the leak on me. I believe rpms do jump when sprayed at base of carb.
 
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