I put a vacuum gauge on my friends 87 GTA because it is not running well after he did the rebuild. At idle the vacuum is fluctuating between 7 and 12 in Hg. We checked each vacuum line one by one and no difference after disconnecting them and plugging the port. When reving the motor it steady goes up to near 20 in HG at about 4000 rpms. At 1000 roms i read it is supposed to be at about 20 in. He put a LPE 216 cam in it which has under .500 inces of lift and 216 degrees duration. At 1000 rpms it is stll not over 12 in. We sprayed car cleaner aound the intake and no change in rpms. What could be the source of the problem?

 

That's a healthy enough cam, but should show more vacuum...have you re-run the valves and checked the ignition out for misfiring/bad timing?

 

We just checked all the hydraulic lifters, they are all the the correct setting, at lash plus one turn. The timing is at 6 degrees witht est disconnected and we advanced the timing to ten degrees with no change, we did not try to retard it, but wouldn't that decrease the vacuum?

The car would also ping at WOT, and it would sometimes diesel when he turned the car off.

When he took the car out for the first time after the rebuild it ran fine. The second time he took it out it through a code 36, so we replaced the maf burnoff relay, which fixed the code. We thought he MAF might be bad, so i put on mine with no results.

 

Run on and ping indicates a vacuum leak/lean condition, or about a dozen other things too

Fluctuating vcauum at idle seems more like a misfire if valves are set correctly...you sure all plug wires are correct? Doesn't hurt to triple check things.

Zero-lash plus a full turn is correct for a stockish-mild cam, more agressive ramps/cams want less lash..might try backing off a 1/2 turn.

 

call cam manufacturer for proper valve lashing, but one full turn is too much i think. half turn as suggested would be better, quite possibly may be holding a valve open causing this vacuum problem.

 

That is what i thought too, i have always heard 5/8 to 3/4 turn, but that is what the service manual says. Also, after taking the valve cover off and putting the motor at TDC of the number one cylinder, it is not too tight on the pushrod, just past lash because i can slightly wiggle the rocker arm, unlike when the valve is being pushed down.

We just unbolted the plenum and runners, the gaskets looked like they were sealing fine, next we will do a compression test and then check the intake manifold gaskets.

Thing is that it ran fine immediately after he finished the rebuild, it wasn't till the second or third time he took it out, and then it throught the code 36, ever since that it has been running poorly. The code 36 could just be coincidental, he also had open headers, so maybe it was knocking and he couldn't hear it the the pistons are bad???????????

 

What's code 36? I don't have manual handy.

 

MAF burnoff relay.

 

Ahhhhh, MAF burnoff could be due to it not turning off, which would make incoming air inaccurate.

Have you tried swapping the BO relay yet?

 

 

That'll teach me to re-read things

If you disable the MAF(electrically), you should see a difference in operation, if it's MAF related. I have a '89 Helms I'll scan through tomorrow and see if anything rnigs a bell.

Funny, I can see a BO function mishap causing drivability issues, but other than the relay, unsure what could cause a problem (since you replaced it and the MAF already)

 

Well, when the piston moves down, it pulls vacuum throughout the intake track. So there has to be a leak somewhere in order to have that low of a vacuum reading. Bad timing or poor AFR could only make the reading off an inch of Hg. or two, i would think.

 

No, erratic timing (misfire/etc) will cause the vacuum to sweep quickly.

A vacuum leak is usually a constant..i/e whatever size hole you have leaking atmosphere into the system, remains the same...now when this happens, the rest of the engine management tries to compensate and you can get a surge (vacuum rising and falling) or if no compensation is made, just a lower reading..like a brake booster unattached), open TB blades, etc.

Erractically going up and down in the reading is due to cylinder fill or poor explosue of the mixture....assuming something isn't chaotic in the spark control...misfire? JMHO

 

Could this reading just be from the cam, it is the LPE 216 cam with 216/219 degrees of duration with .462/.470 inches of lift with a 112 LSA. Although i have seen bigger cammed motors with twice the amount of vacuum.

 

It's not the cam. My cam is slightly larger and I have 15"hg at idle. I think you have a vacuum leak or a missfire. I'd pull all the plugs and take a look at them.

 

Just pulled the plugs and #2 looked like it never fired, did a compression test, and no compression. Everything looked fine, put it back together, and the compression was fine, maybe a stuck valve? He just had the heads redone so i don't know. Or maybe the valve lash was to tight, but i could wiggle both the rockers when the cam was not in lift?