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  #1  
Old 11-22-2009, 09:11 AM
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JeepsAndGuns JeepsAndGuns is offline
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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.
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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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  #2  
Old 11-22-2009, 10:03 AM
ob1jeeper
 
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I'm betting it has a pretty good lope at idle? If so, my first guess would be a cam grind with a bunch of valve overlap.

To calm down the guage for a cam grind like that, try inserting an orficed plug in the guage line. I'd shoot for something like a 0.040" or so orfice...
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  #3  
Old 11-22-2009, 11:06 AM
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68glad 68glad is offline
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Quote:
Originally Posted by JeepsAndGuns
It jumps from one side of the guage to the other super fast.

Wow, I'd think thats not caused by the overlap. Shouldn't jump that far. Here's something to chew on.
http://www.secondchancegarage.com/public/186.cfm
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  #4  
Old 11-22-2009, 06:38 PM
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JeepsAndGuns JeepsAndGuns is offline
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Interesting link 68glad, but none of the little scenarios at the bottom quite matched what mine is doing.

The cam I have is a pretty mild one, comp cams 256H. I think plasticboob has the same cam, and IIRC, he doesnt have any problems. But I will contact him and ask.
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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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  #5  
Old 11-22-2009, 06:47 PM
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I had a '68 302 that did the same thing, ran like a raped ape, never did figure it out.
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  #6  
Old 11-22-2009, 10:34 PM
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Hey, got your PM J&G... I'm sorry to say that I get "Scenario 3" in the link that 68Glad posted. You mean yours swings all the way from one end to the other? How many units does it swing? What is the scenario that best matches your engine?

This cam is a dream for fuel injection; if you get this jumping-gauge problem sorted out then you shouldn't have any issues at all. By looking at the specs, someone would think that the cam has a pretty good lope - but nope! According to Comp Cams, the ramp angles on the lobes are so "Xtreme" that they open and close the valves just fast enough to not let the vacuum at idle get out of control. I was really worried that it wouldn't work with TBI but I said "screw it, I can tune to make up for ANY cam" and lo and behold the ECU worked pretty well even with just the "out of the box" settings.

Have you tried an orifice plug like ob1 suggested? I have also read that an empty fuel filter will work to calm down the reading to the MAP sensor if necessary.

Hmm, would a messed up PCV valve cause this? What about a slightly messed-up firing order? Let us know what you find out, sorry I couldn't be of much help.
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  #7  
Old 11-23-2009, 06:29 PM
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None of them in that link quite match what mine is doing. If there was one remotely close, it would be #6, but the needle moved in a much wider range, and WAY WAY faster.

When I had the intake off, I pulled the PCV valve and checked it, it still rattled and seemed good. Firing order is correct.
I dont have any orifice plugs, but I will try slightly pinching the hose to see if mabey that helps. I may try that dry fuel filter trick and see if that does anything. It will probably be wed before I get the time to fool with it.


One thing I have just thought of. I have a 1in thick plastic insulator/spacer under the carb. It is the 4 hole style and keeps left and right sides seperate. Theres really nothing that allowes the sides to equilize. Could this by some chance be causing this? If I replaced it with a open center spacer do you think it could change?
But thinking, if the carb is bolted straight to the intake, its the same way. So probably this has nothing to do with it.
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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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  #8  
Old 11-23-2009, 06:34 PM
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is the cam timing installed straight up or did you advance or retard it?
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its a jeep thing i dont understand either
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  #9  
Old 11-24-2009, 06:54 PM
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JeepsAndGuns JeepsAndGuns is offline
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Straight up. I lined up the keyways in the cam with the timing set, and lined up the marks on the timing set.
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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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  #10  
Old 12-15-2009, 06:30 PM
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Heh, almost forgot about doing some more checking, but finally fooled around with it a bit sunday afternoon.

I took and unhooked the PCV and plugged it just for craps and laughs. No change, so I hooked it back up.
I didnt have any type of orfice, so I simply started pinching off the hose to restrict it. The more I restricted it, the smoother the needle got (to a point) I could pinch it almost closed and it would hover around 15 plus or minus 1-2. If I pinched it off totaly, it would hold what was in the line, witch was basicly right at 15.
I had the idle set at about 550, so I messed around with it a bit and set the idle up to about 650 and the needle calmed down a bit (but not much) It now bounces super fast between 10 and 20 (where before it was from one side of the guage to the other) So I left it at 650.
If I rev the engine it smooths out ok. I held the throttle by hand a bit and it was around 20 or a little more. I would guess I was around 1500-2000rpm (couldnt see the tach from under the hood) But the needle would still wiggle a bit.

So all and all, I am still confused.
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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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  #11  
Old 12-15-2009, 07:15 PM
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J10_Jimbo J10_Jimbo is offline
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Not sure if this makes a difference, but where are you connecting your vac gauge to? are you pulling stright off the manifold or from the carb?
Jimbo
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  #12  
Old 12-16-2009, 06:26 PM
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coming from a un used port on the intake just to the passenger side rear of the carb.
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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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  #13  
Old 12-16-2009, 08:53 PM
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Try new vacuum fittings in the intake, sometimes they crack, and use a sealer. I use silicone. Also check the carb base plate bolts. A new carb is not necessarily a good one. Are you using a vacuum ball for the heater vacuum lines? Those crack, too. Took me two weeks to find that one. The spikes happening so quick makes me think the leak is really close to the gauge.
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Seemed like a good idea at the time...
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  #14  
Old 12-16-2009, 09:36 PM
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JERSEY JOE JERSEY JOE is offline
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VACUUM READINGS

MY 2 CENTS:

IF IT FLUCTUATES BACK AND FORTH VIOLENTLY I WOULD LOOK FOR A LEAKING VALVE. ALSO MY IN MY EXPERIENCE MOST AFTERMARKET CAMS RUN BETTER IN THE ADVANCED POSITION ( OFFSET CRANK LOWER SPROCKET)
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  #15  
Old 12-17-2009, 06:17 PM
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JeepsAndGuns JeepsAndGuns is offline
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Quote:
Originally Posted by lkmarsh
Try new vacuum fittings in the intake, sometimes they crack, and use a sealer. I use silicone. Also check the carb base plate bolts. A new carb is not necessarily a good one. Are you using a vacuum ball for the heater vacuum lines? Those crack, too. Took me two weeks to find that one. The spikes happening so quick makes me think the leak is really close to the gauge.

No vaccume ball for the heater controlls, that things been gone for years.
All carb gaskets are new, I have replaced them a couple diffrent times just to make sure.
What in the carb could be bad? I got the carb brand new when I built the engine. Its a holly 670cfm truck avenger.
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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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  #16  
Old 12-17-2009, 06:44 PM
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AMC-J/20 AMC-J/20 is offline
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Have you had the valve covers off lately and watched it run ? . . . try it once if not . . .
I had a Ford 351M do this turned out to be a couple of bent push rods

Mike
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Old 12-18-2009, 06:42 PM
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This engine has done this through two sets of cyl heads. When I swapped heads I checked all the pushrods, rockers, etc..
When I pulled the intake this last time to reseal it again, I inspected each one and all looked good.
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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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  #18  
Old 12-18-2009, 06:47 PM
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AMC-J/20 AMC-J/20 is offline
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alright then, that scratches that

Other then the gauge jumps around anything else acting screwy ?
runs n drives fine ?

this might end up being one of them "if it ain't broke don't fix it" situations, tho i am sure its rather annoying


Mike
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1962 GMC C3000.
1969 AMC AMX-390 Resto.
1971 AMC/Jeep J4000DRW.
1983 AMC Eagle project SX/290.
1988 AMC/Jeep project MJ/343.
1989 Jeep GrandWagoneer.
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  #19  
Old 12-18-2009, 09:42 PM
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Hankrod Hankrod is offline
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Pull the plug wires one at a time with it running and see if it has any effect. Or you can just short the wires one at a time to the battery positive terminal and that will prevent the plug from firing. You have something interrupting the steady vacuum signal. Either a bad valve or a out of place combustion activity. To make a vac. gauge move that much, a reversion must be occurring. Are the pushrods the correct length? Did you check the lifter preload when the rockers where installed? do you have adjustable rockers? The heads or block may not be straight and allowing just one of the outer valves/pushrods to run a little tight, and this may not show up in a compression test. Just trying to help. Good luck, Hankrod
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  #20  
Old 12-18-2009, 11:35 PM
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Is the heat riser valve on the exhaust manifold stuck closed or partially closed?
My vacuum readings go from a jumpy 15 to 18 when the engine is fully warmed up and they steady out. 18 was not easy to achieve, but it seems to be worth the effort in terms of throttle response.
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