Engine only pulls 3" hg of vacuum at idle?

ram360 · Aug 24, 2005

Yeah, that's not a typo. I put a gauge on it and it's 3" hg at idle. I put the same gauge on my 88gt in the same spot and pulled a solid 15 at idle. Apparently this is the problem w/ my car with the code 91 (system lean) keeps showing up. People have told me a vacuum leak would cause this code but I figured I would have heard hissing or noticed something. The idle on the car is pretty solid and I didn't even suspect a vaccuum leak however when I put the gauge on there I couldn't even believe it. I checked over the hoses and everything is hooked up and not leaking (the hoses running from the vacuum tree) Anybody have an idea? I'm suspecting the gasket that mates the upper plenum to the lower and probably going to replace it tomorrow. I have a b cam in the car and figured vacuum would be a little lower but this just doesn't seem right.

vristang · Aug 24, 2005

Yeah, that seams really low.

I had the lower intake/head gasket vac leak, and my vac was not any lower because of it. You should be able to look at the gasket between the head and the intake, and see if it has slipped down. It took about .25" of shift in the gasket to cause the vac leak to the crankcase.

The b cam has a lsa of 107 and 224 duration. My cam has 112 lsa and 230/236 duration. I was still able to get 17" at 1k rpm, with oil (and a vac leak through the pcv). I had that 17" both with and after the intake gasket problem. My idle has never surged, even with all of these oddball problems.

Sounds to me like your problem should be elsewhere. Did you use starting fluid to check for vac leaks? Are you getting any oil in the intake? How old is the block? Worn rings will show as a lower vac reading, but probably not that low? Was it difficult to set idle? How far in did you have to set the idle screw?

I would suggest putting the vac/pressure gauge on the crankcase/pcv output. It can tell you alot.

I don't know if I helped, but I do wish you luck. I know exactly how frustrating this kind of thing can be.
jason

Grn92LX · Aug 24, 2005

I had that issue. It was a big vacuum leak. I had a check eng light. I was making 3" at idle too. I had forgotten to plug one of the upper intake vac ports and I had a little leak at the lower too. Now I make around 9-10" with a 950rpm idle.

HISSIN50 · Aug 25, 2005

Take a look at the bigger hose that comes from the charcoal cannister and goes under the front of the upper intake. That hose vulcanizes easily. It should not be too hard to find the leak. Good luck.

giddyup306 · Aug 25, 2005

or look at the IAC. If it runs MUCH better when you unplug it and plug it up......

ram360 · Aug 27, 2005

I replaced my plenum gasket. Plugged all the unused vacuum ports w/ new caps. Checked/ Replaced hoses and I still have nothing actually I'm pulling less than 3" hg. It's more like 1. The motor was completly rebuilt. I just put it back in a month ago. I only put 300 miles on it since then. Like I said the car ran good. Idle is also good. I'm now totally lost. The car is serously pulling hardly any vacuum. I pulled the vacuum hose from the brake booster and I can tell there is hardly any suction coming out of there. I know when I do this on other mustangs it has a good deal of it. I have a charcole canister on the car but It was never hooked up....(actually I'm not even sure what it does?) Also I'm hearing a small pop or something coming from the exaust when it idles. It's very faint but still something that just appeared out of nowhere. I'm about to take it to a garage b/c I've just about exhausted all of my options. I can't find a leak.......yet I'm pulling nearly no vacuum on a freshly rebuilt motor.

Does it sound more like a crankcase vacuum problem to you guys? I'll try getting a reading at the pcv and see what it says there.

ram360 · Aug 27, 2005

also the car was sucking a little oil thru the pcv and ending up in my intake. Not sure what to do about that or if that has something to do w/ the vacuum?

z9_87 · Aug 27, 2005

How much do you pull at higher rpm's?

ram360 · Aug 27, 2005

like 10 or a little over at 2000 rpm

z9_87 · Aug 27, 2005

how about wot? It should be less with increasing rpm.

ram360 · Aug 27, 2005

not sure about wot but it seem like it peaks at like 10" hg at 2000 and doesn't go any higher than that reguardless of the rpm after that.

vristang · Aug 28, 2005

ram360 said:
also the car was sucking a little oil thru the pcv and ending up in my intake. Not sure what to do about that or if that has something to do w/ the vacuum?

To get the oil flow under control you may just need a restrictor on the TB/VC hose. I have a restrictor here which is about 1/8 ID and it is working very well. The idle quality is better and I am getting no more oil in the oil seperator I just installed.

I found a part at Action Auto that worked well as the restrictor. It is actually a male-male vac line splice. It has a couple of sizes that can be used, by breaking off the smaller fittings. The part number is 715000. It is a 5/32 x 1/4 x 3/8 vac line adapter.

This may also help your engine vac, but I don't know by how much?

good luck
jason

ram360 · Aug 28, 2005

vristang said:
To get the oil flow under control you may just need a restrictor on the TB/VC hose. I have a restrictor here which is about 1/8 ID and it is working very well. The idle quality is better and I am getting no more oil in the oil seperator I just installed.

I found a part at Action Auto that worked well as the restrictor. It is actually a male-male vac line splice. It has a couple of sizes that can be used, by breaking off the smaller fittings. The part number is 715000. It is a 5/32 x 1/4 x 3/8 vac line adapter.

This may also help your engine vac, but I don't know by how much?

good luck
jason

I'll have to give that a try. I'll try to pick one up tomorrow. I don't have an action auto around here. Maybe advanced or autozone carries them under the same part #. Thanks

ram360 · Aug 28, 2005

I put the gauge on the pcv output and I'm lucky if I'm pulling 1" hg from it. The PCV is new. I cleaned out the screen below the pcv (the best I could) when the engine was out and reinstalled it. I'll check the vacuum one more time when I'm back from work just to make sure. BTW that reading doesn't change whether I rev the engine or not.

z9_87 · Aug 28, 2005

Does the car drive fine, do you notice anything wrong with it other than the low vac? It seems like a good size vaccum leak, but your car would probably hardly run with that big of a leak.

Grn92LX · Aug 28, 2005

What is your lower manifold torqued to? Ever retorque it? I go 22 ft/lbs on the lower. Keep going through the sequence until all bolts are good.

HISSIN50 · Aug 28, 2005

I did not re-read all this, but are you sure that your vac gauge is calibrated and/or not broken? If you are still SD, you would have a hard time even getting the car to run with so little vac signal.

Good luck.

ram360 · Aug 28, 2005

z9_87 said:
Does the car drive fine, do you notice anything wrong with it other than the low vac? It seems like a good size vaccum leak, but your car would probably hardly run with that big of a leak.

The car actually runs and drives fine. Idles good, revs nice. No missing or hesitation.

Grn92LX said:
What is your lower manifold torqued to? Ever retorque it? I go 22 ft/lbs on the lower. Keep going through the sequence until all bolts are good.

I didn't recheck the lower. I only put 300 miles on it since the rebuild but I will go over them.

HISSIN50 said:
I did not re-read all this, but are you sure that your vac gauge is calibrated and/or not broken? If you are still SD, you would have a hard time even getting the car to run with so little vac signal.

Good luck.

The vac gauge works. I used it on my 88gt on the same port and pulled a solid 15" hg at idle. Tried it on the same spot w/ this car and got 3" or less. The car in question is mass air (89 gt specs). Like I said I didn't even think there was a leak b/c of how good it ran. I can pull a big vacuum line off the car now and it has little or no effect on the engine at all idle barely increases 100rpm...

z9_87 · Aug 28, 2005

I don't know if the engine had only 3" then it would not run right. I think your gauge is off, it has to be.

ram360 · Aug 28, 2005

z9_87 said:
I don't know if the engine had only 3" then it would not run right. I think your gauge is off, it has to be.

Somehow it runs and runs good the way it is. Like I said I have this car right alongside my 88 gt and I put the gauge on one car and then the other (both in the same spot) on pulled 15 at idle and the other pulls 3. I'm going to stop down a garage tomorrow and talk to a mechanic friend. I'll see what he says and maybe I can bring it down for him to look at it. I've tried looking everywhere and just can't seem to locate a problem anywhere.

Engine only pulls 3" hg of vacuum at idle?

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