Low Vacuum - Causes?

[5.0L Miata]

Hey All,

This has been an ongoing saga that I hope you can help me solve. The engine is from a 91 Mustang GT and it has Edelbrock RPM heads, Explorer Intake, 70mm TB and EGR spacer, on a stock bottom end (including cam). I am still running Speed Density from when the car had an 88 engine in it.

I have not been able to get the car to pass an e-test because it has been running too rich. Since playing with it further to try and fix the problem I have made it run even more rich. I am showing only 10 inches of water of vacuum at idle and the car is running so rich that I can only run it for a minute or two before I have to evacuate the garage. While running it almost sounds like it is missing, but all cylinders are firing.

I have pulled the upper intake a number of times and cleaned all mating surfaces including the TB, EGR spacer, EGR valve, and IAC valve and applied vacuum grease and new gaskets to all. I have also replaced all of the vacuum lines with new including new hose clamps. The water temperature sensor is also new and the timing has been set at 10 degrees with the spout connector out.

What else would cause a low vacuum situation? I installed the heads myself and it was my first time setting the pre-load on the rockers and such. If I had them too tight (or too loose) would this cause a low vacuum situation? What other causes should I be looking at?

I have had a number of people tell me to convert to MAF...and perhaps that is the utimate answer to my emissions problem. But I don't think that is going to help my vacuum sitation. On last thing, when I turn the car off I can hear a leaking sound coming from the driver's side of the engine. It sounds like compression leaking out from somewhere and after about 30 seconds it stops. Any thoughts?

Here is a video of the car running. Sorry about the sound quality, I took it on my still camera.

http://video.cardomain.com/Clip.aspx?key=04A40726C30AEDC8

Thanks.

 

[ID89GT]

How many miles were on the bottom end?

 

[5.0L Miata]

How many miles were on the bottom end?

About 90,000 miles. Compression test looked good when I first installed the heads last spring.

 

[white 89 GT]

Do a compression test, if its a little low it could be the rockers are to tight, I don`t know if that would create a rich condition like your saying.

 

[5.0L Miata]

Do a compression test, if its a little low it could be the rockers are to tight, I don`t know if that would create a rich condition like your saying.

I will do again. Will the rockers being too tight create a low vacuum situation? If it would, then having low vacuum causes a rich condition because the SD computer gets thrown off its game with low vacuum and dumps fuel.

I'll do the test and post up my findings...

 

[matts90lxhatch]

I will do again. Will the rockers being too tight create a low vacuum situation? If it would, then having low vacuum causes a rich condition because the SD computer gets thrown off its game with low vacuum and dumps fuel.

I'll do the test and post up my findings...

The rockers too tight will definitely cause a low vacuum problem. I had that happen when I was adjusting the valves on my old 331 once. I had 3 of them too tight and the car would barely idle and had almost no vacuum.

 

[5.0L Miata]

The rockers too tight will definitely cause a low vacuum problem. I had that happen when I was adjusting the valves on my old 331 once. I had 3 of them too tight and the car would barely idle and had almost no vacuum.

Thank you sir...that is the confirmation I wanted. I'll pull her apart and try it again and see if that fixes things. I think I may lean on a friend who has built many engines in his day to make sure I do it right this time.