low engine vacuum

[Frank V.]

I have a 71' 350/270 engine that is driving me crazy. When I first bought this car it ran very poorly at idle. It had low manifold vacuum (13") and would not idle no matter how you set the mixture. The exhaust smelled like raw gas. I gave several mechanics a crack at figuring out what was wrong. None of them could come up with anything except that the engine was tired and needed overhauling. I pulled the engine out and had it rebuilt. The engine was worn, and it needed to be rebuilt, as it turned out upon inspection. But now the engine is back in the car, and it still runs terribly at idle, reeks of gas and has 13" of vacuum, as before. It is not the carb. The carb was tested on a test engine and functions perfectly.
Everything else in that engine is new, except the castings. The machine shop says they inspected/magnafluxed all castings and found no cracks.
Right now I have all vacuum ports plugged and a gauge on the manifold tee. I have verified tdc of piston at 0 degrees. I have verified cam timing is correct. Ignition timing is set a 8 degrees btdc. I have tried spraying propane around the base gasket and the intake-to-head gaskets. What am I missing? Is it possible to have a crack in the intake somewhere else that could cause this problem?

[Edward J.]

Re: low engine vacuum

Frank, Maybe What You Have Is A Carb Problem, To Much Fuel While Running At Idle Will Cause You Eyes To Burn And Make It Run Poorly , Causes Can Be Float Level In Carb To High, Idle Circut Metering Jets Worn Badly,idle Mixture On Carb Not Set Correctly ( This Being The Easiest Fix)

[Duke W.]

Re: low engine vacuum

Engine idle vacuum is very sensitive to valve overlap, so the first question is what camshaft came out and what was put back in?

Is it a manual tranmission or TH 400? You need to specfiy the exact conditions that you are measuring manifold vacuum.

1. XX"@XXX RPM idling in neutral (manual trans)
2. XX"@XXX RPM idling in Drive (TH400)

Duke

[Frank V.]

Re: low engine vacuum

This is the 4th carburetor installed on this engine. None of the symptoms change. Idle mixture screws do nothing until you turn them in all the way. Then the car dies. The current carb and the one before it were tested on another engine and performed perfectly. It still doesn't explain the low vacuum issue. I can't see how a carb could cause this problem.

[Joe L.]

Re: low engine vacuum

This is the 4th carburetor installed on this engine. None of the symptoms change. Idle mixture screws do nothing until you turn them in all the way. Then the car dies. The current carb and the one before it were tested on another engine and performed perfectly. It still doesn't explain the low vacuum issue. I can't see how a carb could cause this problem.

Frank-----

First of all, I think you made a big mistake just having the engine overhauled on the basis of someone's supposition that, failing all other attempts at correction, you had a worn out engine. You should have performed a cylinder leak-down test or, at least, a compression test. That would have allowed you to determine if there was a problem with the core engine. If that had all checked out, then you WOULD NOT have required a complete engine overhaul and the problem would have to have been elsewhere. But, what's done, is done.

By using the carb on another engine for which it performed satisfactorily, that pretty much rules out the carb as a source of the problem.

At this point, I think I'd be looking for a possible vacuum leak somewhere besides the usual engine-oriented locations. Have you tried removing the external vacuum connections (i.e. to vacuum tank and power brake units) and plugging the manifold vacuum fitting which supplies these systems with vacuum?

Another possibility is that you have a severe "mismatch" of the intake manifold-to-cylinder head mating surfaces. This can create a vacuum leak at EITHER the BOTTOM or the TOP of the manifold-to-cylinder head interface. However, this is not something that would just "spontaneously" develop-----it's either been there since the car was new (unlikely, since the original owner would have complained about it long ago), or it resulted from some other work done on the engine (e.g. change of cylinder heads, change of intake manifold, surfacing of cylinder heads, surfacing of intake manifold, change of block). If such a problem was created at any time, it won't be "rebuilt out" of the engine when you had it overhauled; it will remain with the engine. Do you know if any of these had been replaced prior to the development of your problem? Do you know if the intake manifold or cylinder heads were worked on prior to the problem?

Yet another possibility is a cracked intake manifold or cylinder head. Did the shop check these for possible cracks?

[Frank V.]

Re: low engine vacuum

This car has an th400 transmission. At 950 rpm it is producing 20" of vacuum. It wont idle below that reliably. It just coughs and sputters and the vacuum falls off to about 13-14"s. The cam that came out of the car was stock, the one that went in was a reproduction gm 300hp cam. I have called the manufacturer and they have assured me that the cam is not the cause of the low vacuum.

[Timothy B.]

Re: low engine vacuum

Frank,

Is there some kind of vacuum modulator that goes to the transmission or did you plug that as mentioned in your first post? What camshaft manufacturer did you purchase the cam from? I once had a 72 low HP W/ TH400 vette that I changed the cam to the 350/350 HP cam and never got the car to idle correctly with the T/H 400. I think the car ran better with the old cam.

I always thought it had something to do with the stock torque converter in the transmission.

[Frank V.]

Re: low engine vacuum

The engine had low compression, was burning oil and had no power. When the engine was diassembled, it was found that the had been rebuilt before. Unfortunately, the person who did the rebuild tried to put 30 over pistons in the engine without boring it first. All piston skirts showed severe wear. One of the crank journals was so messed up that the crank required replacement. The camshaft was severely worn. The machine shop says that all castings were magnafluxed and inspected for cracks. None were found. The block was lightly decked , and the heads were resurfaced. I will ask the machinist about the intake alignment issue. How would that kind of problem be verified though? I have also heard that possibly the exhaust crossover is leaking to the plenum, a defect that would be difficult to prove also, and I was wondering if anybody else had ever had the same symptoms.

[Roger S.]

Re: low engine vacuum

When I got my 73 L-82, AT, I could only measure 8-1/2" Hg at idle, the car would not idle smoothly when in drive. After a year or so and much checking I measured the vaccum at the oil dipstick tube and measured 8-1/2" Hg. There should be none there. Removed the intake manifold and reinstalled it and the vaccum went to 13-1/2" Hg and the car would idle smoothly.

Roger

[Duke W.]

Re: low engine vacuum

This car has an th400 transmission. At 950 rpm it is producing 20" of vacuum. It wont idle below that reliably. It just coughs and sputters and the vacuum falls off to about 13-14"s. The cam that came out of the car was stock, the one that went in was a reproduction gm 300hp cam. I have called the manufacturer and they have assured me that the cam is not the cause of the low vacuum.

I'm not trying to beat you up, but you still have not listed enough useful data. Is the 20" of manifold vacuum at 950 in Drive or neutral? How does the idle vacuum behave as you lower idle speed in Drive - specific data like "in Drive idle vacuum falls to 16" at 800, 14" at 700, then the engine stalls", for example.

If you have the manufacturer/part number of the camshaft, some on this board may be able to verify if it is indeed the correct OE grind.

I am seeing an increasing number of posts were the writer says "my mechanic says it's okay" and "the parts guys says all the parts are right" and no useful test/configuration data is provided, and we are asked to diagnose a problem. Garbage in, garbage out - if useful test and configuration data is not provided all anyone can do is guess, which may just send you on a wild goose chase.

The more useful information you provide in your post, the more narrow and likely accurate the diagnosis will be, but don't be surprised is the answer is "run the following tests/diagnostic procedures" to narrow down the possibilities.

C3s have a lot of vacuum systems, and it's easy to connect hoses to the wrong ports when reconnecting everything after major work like an engine overhaul. All available references such as the CSM and AIM should be consulted and a thorough check of all vacuum hose routing/connections should be done to ensure that all hoses connected to the correct nipple. I've seen simple hose configuration errors cause problems for years and the owners spent hundreds of dollars trying to fix a problem than was nothing but something as simple as a couple of mixed up vacuum hoses.

One way to isolate a leak is to use a clamp to sequentially clamp off all vacuum supply hoses (including the power brake booster hose). If engine idle characteristic changes as you clamp off a vacuum supply hose, you have located a possible system leak. The PCV and EVAP systems configuration/function should also be checked whenever idle problems occur. For example, mixing up the EVAP cannister signal and purge hoses can cause a significant vacuum leak at idle. The EVAP cannister signal is from a "ported" vacuum source, so the purge valve should not open until the throttle valves are above the normal range of idle position. If these hoses are mixed up the purge valve may be open at idle, which effectively creates a signficant vacuum leak.

Use a systematic approach and keep a cool head.

Duke

[John D.]

Re: low engine vacuum

Duke, I agree with you 100%. Here is a quicky story of what just happened to one of my customers. He had his 60-61 engine rebuilt by a shop that swears they put in the correct replacement fuel injection cam. As you know it should have the Duntov 097 cam. But what did they do to the poor owner. The rebuilder put in a fuel injection 30-30 cam. Course the rebuilder said it was the real deal. What a shame. JD

[Jim T.]

Re: low engine vacuum

Frank you mentioned in your first post about having all the vacuum ports plugged.
With all you have done to find the low vacuum at idle with all ports plugged recommend two things. Connect your distributor vacuum cans port directly to the carb's full time vacuum port. Also verify that your distributor vacuum can is not ruptured and holds vacuum.
Remove your distributor cap, remove the distributor's rotor. Disassemble the centrifical advance springs and weights and clean it all. I very lightly lubricate my 68 and 70's pivit posts and where the weights slide with distributor point cam lubricant although it did not leave the factory with any lubrication here.

[Joe L.]

Re: low engine vacuum

The engine had low compression, was burning oil and had no power. When the engine was diassembled, it was found that the had been rebuilt before. Unfortunately, the person who did the rebuild tried to put 30 over pistons in the engine without boring it first. All piston skirts showed severe wear. One of the crank journals was so messed up that the crank required replacement. The camshaft was severely worn. The machine shop says that all castings were magnafluxed and inspected for cracks. None were found. The block was lightly decked , and the heads were resurfaced. I will ask the machinist about the intake alignment issue. How would that kind of problem be verified though? I have also heard that possibly the exhaust crossover is leaking to the plenum, a defect that would be difficult to prove also, and I was wondering if anybody else had ever had the same symptoms.

Frank-----


I'm sorry that I missed the fact, stated in your original post, that you had removed the vacuum connections and plugged the vacuum fitting on the manifold. That pretty much eliminates those as a source of the problem. Still, if you haven't already, I would check the distributor vacuum control as Jim mentioned.

The previous engine rebuild sounds like it was done by the "ultimate bubba". However, I don't think they tried to use 0.030"-over pistons without boring the engine. I don't think there's any way you could get such pistons into a standard bore, even if the bores were worn. So, if it had 0.030"-over pistons in it, someone must have done some sort of a "bore job".

The intake-to-head issue that I described earlier is very difficult to assess and even more difficult to correct. However, if you have 20" of vacuum at 950 RPM as you describe, I don't think that's your problem.

For some reason, I was thinking this car was a 4 speed. However, now that I see that it's a THM-400, I think that a problem with it could well be the root of the problem. I think I'd look into that.

It's possible that incorrect components were installed in your engine during the latest rebuild. However, it seems very unlikely to me that, even if that occurred, those incorrect components would cause exactly the same problem as existed in the engine prior to rebuild----possible, but highly unlikely.

[Frank V.]

Re: low engine vacuum

Here's the thing. With NO vacuum being taken from the intake manifold or the carb AND all carb ports plugged, i still only get 13-14" vacuum at 650 ( the lowest speed I can idle it down to). The distributor was rebuilt as well during the overhaul (because of a worn upper bushing) which included a new advance vacuum actuator.

[Frank V.]

Re: low engine vacuum

The transmission was completely overhauled with the engine. It was leaking and had clutch material in the pan so I had it done at the same time. Also, when I tested vacuum it was with the tranny modulator line disconnected, just in case the trans was causing the problem.