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Correct Vacuum reading

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I have owned this car for years, 1993 Convertible.

The last two has been a nightmare. I kept telling myself I would get around to fixing it.

I believe I have a vacuum leak. Sprayed carb cleaner all around and no difference.

Last year the tranny would not shift out of second. Pulled and rebuilt the tranny.

My guy said it was a little worn, but nothing that would prevent it from shifting.

He felt it’s a vacuum issue with the modulator.

Put the new tranny in and same issue.

Started looking at the vacuum.

Removed and plugged the vacuum lines around the MAP sensor and checked the vacuum. 8 inches of vacuum.

If this was a carbed engine, it would barely run.

What should the correct vacuum reading be?

We are going to do more testing tomorrow, but would like to know what it should be.

Usually at idle I expect to see 15 inches of vacuum. Does that hold true to this engine?


3 years ago we had to fix the leak on the plug for the distributer hole. Meant taking off the heads which meant removing the timing belt.

I bought the tool and I believe the timing is correct.

Here is the plan of attack:

First, what is the correct Vacuum reading at idle?

Plug ALL vacuum ports at the engine to insure there are no external leaks.

If vacuum not right, spray with carb cleaner to see if there is a vacuum leak.

If none found, remove the plenum.

Once removed, do a compression test and a leak down test.

If the valves are out of time, I should see a poor leak down test.

If it fails, I now found my problem.

Insure the plenum has no issues and install with new gasket.


Are there any other items I should be looking at?

The mounting for the MAP sensor has the vacuum ports for the tranny, inside vacuum.

It’s plastic. Can this crack?

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A bad vacuum modulator will not cause a no shift issue. It controls the firmness of the shifts based on vacuum. If the modulator is bad or the vacuum supply to it is, it will shift a lot harder under normal driving conditions. Get a new "trans guy" asap.


As for vacuum, 8 in/hg is far too low, unless you're at some crazy altitude. Use that same gauge on another known good running vehicle to make sure you're not chasing your tail over a bad vac gauge.  18-22 in/hg is about normal. Subtract around 1 in/hg per 1,000 ft rise in altitude.

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I have several vacuum gauges. I will verify. But it appears to match the way the engine is running. Vacuum that low even effects the power brake booster.
It pulses as the RPM's change because the vacuum will shoot up with high rpm's and foot off the throttle. Which we see on the gauge. 

When accelerating the gauge goes to 0, which is what I expect.
If the timing belt was off on one side by say a tooth, would the engine run? I am leaning toward that. But I will do all my tests before going there.

A leak down test would find that. 

It does have plenty of power. 
On the transmission, what makes it shift? I thought this was a dumb transmission. 


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The DOHC will pull approx 16" vacuum at idle.

Vacuum ports on the intake are located

center rear of upper plenum for the booster

right rear of plenum for the MAP which supplies a source for the trans modulator & the FPR as well the HVAC controls.

2 ports behind the throttle body for the EVAP solenoid & the PCV, all of these access points make use of molded tubing with rubber ends at both ends.

Check them all at both ends.....and have a close look at the tubing for splits/cracks.

The EVAP solenoid is normally open, the PCM closes it at idle, if the solenoid is stuck open that will cause a problem at idle.

Have a look at the EGR gaskets as well, a leak there is a vacuum leak.


Just out of curiosity sake how did you go about replacing the timing belt?

by the book with the cam cog pullers & rotating the engine multiple times for each cam before setting the cogs?



Edited by 55trucker

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Some people swear by using a Propane Torch (NOT lit) to check for Vac leaks...  You can get to hard to reach areas under the plenum much easier... Never used propane myself,, BUT,,, I have had lessl than desirable results with carb/brake cleaner...

Just an alternate suggestion...

Good luck...

Tom B...

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1.  Before you do ANYTHING else, TEST THE CRANKING COMPRESSION of at least one cylinder on each bank.  You screw up the cam timing, you'll screw up the cranking compression.  Good cranking compression (not too high, not too low) almost certainly means the cams are reasonably close.  This is not as certain on an engine with separate intake and exhaust cams, but it's a good starting point.

1b.  Having the cam timing off may or may not affect the leakdown.  The only way cam position would affect leakdown is if the cam is so far off that either the intake or exhaust valves are open at TDC compression.  If they were, the engine wouldn't run on that bank.  I have heard of guys screwing up the cam timing procedure so badly that the companion cylinders fire at the same time.  Hard on the ignition system, hard on the engine...but possible given a waste-spark system, and all six cylinders "run", just not properly.

2.  As said, lack of vacuum to the modulator won't prevent the trans from shifting, but the shifts will be harsh and at high RPM/speed.

3.  If you're having trouble getting appropriate results checking for vacuum leaks with BRAKE cleaner, it may be because some brands of brake cleaner are not flammable.  I use aerosol CARB spray, I've never run across a carb cleaner that wasn't flammable.  Propane works too.  Aerosol carb spray is easier for me, but can damage painted surfaces.

3b.  BRAKE cleaner that burns may generate POISONOUS GASSES (Phosgene and/or hydrogen chloride) that can well and truly fukk you up.  This has been discussed on welding forums, where guys spray brake cleaner to degrease metal they intend to weld--and end up in the hospital with REAL problems.


Edited by Schurkey

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As for how we replaced the timing belt, I bought two Kent-Moore alignment tools. We followed a 1994 factory manual I have.
A little more background.
The engine would idle OK most of the time, but sometimes it would idle very rough and would cut out. I mean it would really rock and roll.
Starting it afterward would be difficult and would have to crank the engine.
You would be at a light, engine idling fine, hit the gas and the engine dies. Very dangerous. In this scenario, I would place it in neutral and it would start right up. 

I have a 1994 manual because we used to own a 1994.
I have replaced the EGR earlier.

I ordered all of the 1993 manuals on Friday. Next week I have a boat show to get ready for, so not much will be done on her.

Thank you for the value on the vacuum reading.

Tried both carb cleaner and starter fluid, nothing. 

The plan of attack:
1. Forget about the transmission for now, it has no bearing on this issue and will be trouble shoot after this issue is resolved.
2. Verify our testing to date to valid. 
3. Remove the plenum, do a visual and then a compression and leak down test.
 .......If the leak down test shows the cylinder cannot hold pressure, I know what head is giving me the issue.
4. Remove the front of the engine and check the timing marks and re-align if needed.
5. If that does not fix it, go buy a 1999-2002 Buick with a 3.8 and flip it in.

My wife loves the Olds. It's comfortable, good fuel economy, and the only one like it around.
Has anyone ever swapped a 3.8L into this body.
I would want to swap the entire drive train.

I am very tired of these 3.4. Replaced the engine in the 1994 once. This one has always had issues. GM did a poor design on this.





Edited by narnone

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