94 K1500 4.3 low vacuum

[mastersat1]

How do i find internal vacuum leak? I have 14-15" of vacuum with a shaky needle. Motor has 135-140# compression on all cylinders. This motor is a replacement long block with salvage block. I had to use old intake in order to use tbi instead of cpi that came on salvage block. I replaced head gaskets while engine was out. Motor is running rich and my understanding is that low vacuum will cause high voltage on map sensor which will trigger it to run rich. Map voltage is high, 4.5v at idle. Can't find any external leaks. I sprayed a little starting fluid in the valve covers to see if guides were leaking and that didn't change the idle. Has new egr valve and solenoid. Have plugged all ports to check amount of vacuum from motor. I ran a little hotter plug in it and the problem was not as dramatic. Will hotter plugs increase the vacuum? Or am I just compensating and burning the extra fuel. I don't know year of the block for correct plugs. Need to know how to isolate the vacuum leak. Would greatly appreciate any help.

[Technocrat]

Originally Posted by mastersat1

How do i find internal vacuum leak? I have 14-15" of vacuum with a shaky needle. Motor has 135-140# compression on all cylinders. This motor is a replacement long block with salvage block. I had to use old intake in order to use tbi instead of cpi that came on salvage block. I replaced head gaskets while engine was out. Motor is running rich and my understanding is that low vacuum will cause high voltage on map sensor which will trigger it to run rich. Map voltage is high, 4.5v at idle. Can't find any external leaks. I sprayed a little starting fluid in the valve covers to see if guides were leaking and that didn't change the idle. Has new egr valve and solenoid. Have plugged all ports to check amount of vacuum from motor. I ran a little hotter plug in it and the problem was not as dramatic. Will hotter plugs increase the vacuum? Or am I just compensating and burning the extra fuel. I don't know year of the block for correct plugs. Need to know how to isolate the vacuum leak. Would greatly appreciate any help.

I had a similar problem on my 88 Astro. The vacuum was leaking around the throttle plate shaft on the TBI. I bought a gasket set at on line & that fixed the problem.

[MT-2500]

What is vacuum at idle and at 2K rpm?
Is needle holding steady?
Is engine timing good.
Set in bypass mode?

Have you checked egr valve for leaking or holding open?
Pcv Valve good?
Vacuum brake booster leaking vacuum?

[jdmccright]

Spray the carb cleaner all around the vacuum lines, base, and butterfly shaft of the TBI. When the engine stumbles, you found the leak. Good luck!

[tblake]

To check for an internal vaccum leak you pull the PCV valve out of the valve cover, leave it attached to the vacuum source, plug the PCV with your finger, and shoot some carb cleener into the valve cover through the PCV opening.

Are you sure the camshaft timing is dead on? 135-140 seems rather on the low end for compression. Was this motor rebuild before it went in?

[mastersat1]

I sprayed everything externally, intake gasket, vacuum lines power booster hose and check valve. I also pulled pcv valve and oil fill cap and sprayed in valve covers. No change in idle.

[mastersat1]

Idle vac. is 14-15" with a shaky needle. Base timing is set at 0 deg with connector unplugged. Will check vac at 2k rpm tomorrow and let you know. What is "set in bypass mode"?
It was a salvage motor, was told ran good. When I pulled the plugs and the exhaust manifolds all looked good. The bottom end hasn't been apart, so I didn't see any reason for cam timing to be off. Wouldn't that show up in compression or piston/valve clash. I truly thought the compression was good. I though cylinders just needed to be within 10% of each other. I greatly appreciate your help guys. What is the possibility that the tbi manifold isn't mating as good as the cpi manifold did? I was told that it could be oil rings because that is what creates the vacuum. There's no smoke except black from loading up and running rich.

[j cAT]

Originally Posted by mastersat1

How do i find internal vacuum leak? I have 14-15" of vacuum with a shaky needle. Motor has 135-140# compression on all cylinders. This motor is a replacement long block with salvage block. I had to use old intake in order to use tbi instead of cpi that came on salvage block. I replaced head gaskets while engine was out. Motor is running rich and my understanding is that low vacuum will cause high voltage on map sensor which will trigger it to run rich. Map voltage is high, 4.5v at idle. Can't find any external leaks. I sprayed a little starting fluid in the valve covers to see if guides were leaking and that didn't change the idle. Has new egr valve and solenoid. Have plugged all ports to check amount of vacuum from motor. I ran a little hotter plug in it and the problem was not as dramatic. Will hotter plugs increase the vacuum? Or am I just compensating and burning the extra fuel. I don't know year of the block for correct plugs. Need to know how to isolate the vacuum leak. Would greatly appreciate any help.

vacuum should be 15-21 inches at idle. you are a little low.
use you vacuum guage with a T fitting to check for loss of vacuum..

do not use carb cleaner. this will destroy your rubber vac lines and other non metal parts. always use propane gas ..simply attach a hose to the torch head of the small hand held tank. open the valve very little then use the open end of hose to probe for leaks ..a rapid rise in idle/rpm will indicate a leak...

rapid vacuum fluctuations mean valves are bad, this can mess up vac...too much you need to find out why they are not working correctly.

the map sensor on a good engine with a steady idle at 18inches of vacuum the voltage at map will be 1.2-1.5 volts dc..

4.5volts I would replace the map or it is very possible the map O ring is missing ...

[tblake]

It would just be odd for a bad valve because compression between the cylinders are all about the same. How could all six cylinders have at least one burnt valve that would cause compression values all the same?

So to the OP, you took a vortec motor, took the LIM off, and put the old TBI lim back on? What LIM gaskets did you use? TBI LIM gaskets, or Vortec LIM gaskets?

Does the motor run any differently with the MAP sensor unplugged?

[mastersat1]

I bought gaskets for the original tbi setup. Unplugging the map makes no difference. I rechecked vacuum today and cold idle is at 12", at operating temp vac is 13" at idle and almost 18" at 2k rpm. I did notice the vac drops off some before increasing with acceleration. This maybe normal or should it only increase with accel. I read somewhere online that low vac will make high voltage on map so I think my whole richness problem stems from the low vac. I used propane on it and couldn't find any leaks externally. I also stuck the hose in the valve covers and no diff. If I pull the intake back off how can I tell if its not mating right with the heads or the block? If it is leaking internally it would between the intake and heads wouldn't it. Should I permatex the intake gasket? Is there any way of testing for internal leaks?

[j cAT]

Originally Posted by mastersat1

I bought gaskets for the original tbi setup. Unplugging the map makes no difference. I rechecked vacuum today and cold idle is at 12", at operating temp vac is 13" at idle and almost 18" at 2k rpm. I did notice the vac drops off some before increasing with acceleration. This maybe normal or should it only increase with accel. I read somewhere online that low vac will make high voltage on map so I think my whole richness problem stems from the low vac. I used propane on it and couldn't find any leaks externally. I also stuck the hose in the valve covers and no diff. If I pull the intake back off how can I tell if its not mating right with the heads or the block? If it is leaking internally it would between the intake and heads wouldn't it. Should I permatex the intake gasket? Is there any way of testing for internal leaks?

since the compression is good, the low vacuum now much less than 15 is most likely an intake manifold leak ..

if the vacuum is rapidly fluctuating which to ME means all valves are worn/not seating correctly....causing some exhaust to go back into the intake...

did you do any work on the heads when you worked this engine ?

stick the propane tester hose into the air cleaner intake , does it rapidly increase the RPM ? if so then retest for leaks.

[mastersat1]

The vacuum isn't fluctuating rapidly. I shakes about a 1/2". No work to the heads, just gaskets. I just put propane to the tbi unit and it bogs down. Thank you for your time.

[Technocrat]

Originally Posted by mastersat1

I bought gaskets for the original tbi setup. Unplugging the map makes no difference. I rechecked vacuum today and cold idle is at 12", at operating temp vac is 13" at idle and almost 18" at 2k rpm. I did notice the vac drops off some before increasing with acceleration. This maybe normal or should it only increase with accel. I read somewhere online that low vac will make high voltage on map so I think my whole richness problem stems from the low vac. I used propane on it and couldn't find any leaks externally. I also stuck the hose in the valve covers and no diff. If I pull the intake back off how can I tell if its not mating right with the heads or the block? If it is leaking internally it would between the intake and heads wouldn't it. Should I permatex the intake gasket? Is there any way of testing for internal leaks?

I am not going to tell you you "should" Permatex those gaskets (there are a lot of legitimate arguments against it), but I have done so on two 4.3's. Both had experienced severe water leaks due to corrosion. (I know the manifolds should have been replaced, but that was not an option financially at the time.) Neither has had any ill effects. If you do go this route;
1. Use the red (non-hardening) high temp version.
2. Apply sealer to the manifold, not the head.
3. Don't get carried away applying the sealer. Excess can get into the oil causing major engine damage.
4. Keep dirt out of the sealer & let it get a little tacky but not fully dry before reinstalling the manifold. This will help keep excess from squeezing into the engine.

[j cAT]

Originally Posted by mastersat1

The vacuum isn't fluctuating rapidly. I shakes about a 1/2". No work to the heads, just gaskets. I just put propane to the tbi unit and it bogs down. Thank you for your time.

the vacuum fluctuating 1/2in is normal on a 6cyl engine.
when I say rapid I mean the cycle rate is that of the valve train..not a wondering deflection ...

since the rpm droped when you injected the propane this is not normal...I gotta think about this one !


make sure the timing is dead on correct..

[tblake]

The Gaskets for the two motors look about the same. Aside from subtle differences which shouldn't make a difference. (at least that I can see). Only thing I really see different is that on the TBI set of gaskets, there is no seperater between two intake runners on each head. Would you think that might make a difference?

94 K1500 4.3L TBI V6 Fel-Pro Brand Lower Intake Gaskets



96 K1500 4.3L Vortec V6 Fel-Pro Brand Lower Intake Gaskets (upper set included)