Diagnosing Vacuum Leak


Jul 13, 2001
Hey All;

What is the best way to diagnose a vacuum leak? Here's my situation (and I certainly plan to investigate this further with a scan tool and the assistance of a local guru who does much of the work on my car). Just thought I could putter around with it myself in the meantime and maybe learn something.

I suspect, but can't be sure, that the leak could be coming from the plenum. The install of the RJC powerplate makes me think that I may have botched one of the gaskets on the plenum.

Is there a tell-tale way of diagnosinga vac leak? I don't see any black discharge on the lip of the plenum. I do get a tip-in stumble on the car after it's hot, but it's not terrible and it seems to come and go. I did get a code 44 for a couple of weeks after my 009's, TA51 and chip were done, but it went away about a week later and never showed up again. and it would only blow the code when it was idling in park (idle would suddenly change and the code would show up). As soon as it got throttle it would go away.

Car runs like a rocketship otherwise. But the presence of that code 44 still makes me curious. fyi - I run 105 RFG unleaded.

Lastly, since I'm now immersing myself in scantool technology (turbolink), is there something that I can look for that will give an indication of a possible vac leak?

Any thoughts?

combo is below:

TA51, 009's, RJC PowerPlate, Mease Perf. I/C, Walbro 307 h/w and Accufab Regulator, Big Mouth cold air, etc.
This is a very basic approach but I feel it's important. If you can rub a vacuum line between your fingers and get alot of black residue remaining on your fingers afterward (I mean real black!), then the line is dryrotted and highly suspect. I replaced all my vacuum lines last year and most of them were cooked. Outside of that, carb cleaner sprayed on most vacuum connections will cause a rise in idle speed when the leak is found. If you are getting intermittent drops in idle, perhaps a look at IACs or MAFs could source down your situation as well.
Thanks John.

Fortunately, my lines are still soft as the car only has 20k miles on it. Somehow, climate-controlled storage and regular starts/short drives has prevented them from drying and cracking. I didn't think the carb cleaner trick would work around the plenum, but I'll give that a shot :)

What confused the whole issue was that code 44. I saw a thread a while back about how a vac leak could throw this code (haven't changed my O2 sensor yet, but I don't suspect it's bad) so it got me thinking....

Last question: If I remove the vacuum block (I think I'm going to replace the gaskets above and below my RJC powerplate), am I going to be plagued with leaks forever or is it something that generally doesn't present a lot of problems with a removal and install? I do have a new gasket to use.....

Spray carb cleaner around the base of the plenun...if there is a vacuum leak, the cleaner will be inhaled and the engine will speed up for a second.
Rich, being as you didnt have this problem before the injector and chip swap. Theres a good possibility I ****ed up the cal. Like I siad back in November I wanted to look at it. Now if you WANT to try to tackle it, do it right right way, with the right equipment. You need to get in there and see WHY its setting this code. Is it in fact lean? Is the O2 dead? Is there a vacuum leak? Is there a post MAF air leak? Is one of the injectors bad? Is the chip a piece of crap? Looking at O2 activity vs tailpipe emissions is the easiest way to see whats what. Lets get together one evening when its nice out and get this thing figured out ok?

Of course if you WANT to try to figure it out, I'd be glad to help. If you got a laptop I can loan you direct scan. I have a spare ECM all set up already. This way you can post files for more opinions. Actually, I have a spare laptop that I guess I can loan you too, since I now how you take care of things.... ya damn perfectionist. :) Its just the battery only last an hour or so.

Its up to you dude.
Hey Jim :D

Thanks for the suggestions. Yeah, I guess I'm a bit obsessive :) I drove the car for 10 miles the other day and it took me an hour to clean it when I was done :D

Yes, I think I'll wait and enlist your assistance on this one. I think I'll probably learn more if we just get it on the scanner rather than searching for an errant vacuum leak that may or may not be there by spraying carb cleaner all over the place. :)

Just let me know when you can carve out some time :)