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Old 11-22-2009, 09:11 AM
JeepsAndGuns's Avatar
JeepsAndGuns JeepsAndGuns is offline
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Join Date: Jul 18, 2003
Location: Cullman, AL
Posts: 4,575
At my wits end on this funky vaccume reading!

I have been fighting a funky vaccume reading on my cherokee for the past few years since I noticed it. Engine is a 401 rebuilt a few years ago. Intake is a edlbrock non egr preformer, carb is a holley 670cfm truck avenger. All were brand new when I installed them.
I can hook up a vaccume guage and the needle does not hold steady. It jumps from one side of the guage to the other super fast. Everyone tells me vaccume leak. I can rev it up a bit and it steadys out.
I have tried new carb base gaskets, check for leaks with carb cleaner sprayed at all the gaskets. I have removed and installed new gaskets on the intake a couple diffrent times, thinking it might not have sealed, and had a leak on the underside of the intake. I only have two vaccume lines hooked to the intake. The one for the vaccume advance, and the one for the heater controlls. I can disconnect the heater controll line and plug it, and reading does not change.
Another person suggested a burnt valve. And about a year ago, I had a valve spring break on me. (re used original ones when rebuilt)
So I took this as a chance to do some work. I took and found me a good set of used heads. Had them hot tanked and magnafluxed. I then took and did some mild port/polish/gasket matching to them. I then had a 3 angle valve job done, shaved about 20 thou, and capped them off with a brand new set of edlbrock valve springs. Installed everything, and went annal with sealing the intake.
NO change in vaccume reading. (but did get a noticeable power boost after I re tuned everything)
I have changed adjustment on the mixture screws, played with timing, you name it, I have probably tried it with no luck. I still can not get a steady vaccume reading.
I just (like last week) took the intake off again and took it to the machiene shop, thinking it could be warped. I had him check it out and he said it all checks out nice true and even. I asked for suggestions, and he said try a compression test.
Did that yesterday. The lowest was 179psi, the highest was 181psi.
I have thought it could be a bad vaccume guage, so I took and hooked it to my YJ, and it holds a nice steady reading.

So how does the engine run? It runs great. I can burn the tires if I want. So why worry do you ask??? Well I am getting ready to convert to TBI and I am afraid the funky vaccume reading it gonna wreak havok on the MAP sensor readings at idle and make the computer throw codes and not run worth poo.

I am at my wits end on this.
__________________
79 Cherokee Chief 401/T18/D20, MPFI fuel injection, hydroboost, otherwise stock.
Future mods: Caddy 500/NV4500/NP205, HP D60 front D60 smooth botom rear, 5.13 gears, 35x12.50's on H1 beadlock wheels. Warn M12000 winch.
93 Wrangler 4.6 stroker/AX15/NP231,SYE,CV, OME 2.5 lift, front hub conversion/big brakes, 31X10.50's Warn M10000 winch.
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