No vacuum pressure at idle

[doober75]

I just recently bought my first love back, and today I took it out for a quick spin after work. At about 28 psi I heard a loud "pop" for lack of better word. I couldn't tell if it was a backfire or hose blowing off.

After I backed down, I noticed my AFR dropped and car starting running horrible. I nursed it for the 1/2 mile or less to my house and once parked I noticed I had no vacuum pressure at idle. I see that at idle I should be around 19 psi. Now I checked to see if anything was visually blown off and saw nothing. No smoke from the exhaust, no oil leaking anywhere and still have good oil pressure.

Any advice on where to start? I am relatively new to the dsm world but have been reading up and learning slowly. Should I buy the boost leak test kit or am I in to bigger problems?

Thanks in advance guys.

[9!'clipseDOHC]

Originally Posted by doober75

I just recently bought my first love back, and today I took it out for a quick spin after work. At about 28 psi I heard a loud "pop" for lack of better word. I couldn't tell if it was a backfire or hose blowing off.

After I backed down, I noticed my AFR dropped and car starting running horrible. I nursed it for the 1/2 mile or less to my house and once parked I noticed I had no vacuum pressure at idle. I see that at idle I should be around 19 psi. Now I checked to see if anything was visually blown off and saw nothing. No smoke from the exhaust, no oil leaking anywhere and still have good oil pressure.

Any advice on where to start? I am relatively new to the dsm world but have been reading up and learning slowly. Should I buy the boost leak test kit or am I in to bigger problems?

Thanks in advance guys.

Boost leak test. Is your boost gauge showing zero boost as well or just zero vacuum? This is an aftermarket boost/vacuum gauge, right?

[LiquidX]

Loud pop usually indicates a coupler blew off. A boost leak test will find the problem.

[doober75]

Originally Posted by 9!'clipseDOHC

Boost leak test. Is your boost gauge showing zero boost as well or just zero vacuum? This is an aftermarket boost/vacuum gauge, right?

Yes it is just sitting at zero, and is an aftermarket gauge. I forget the brand off hand to be honest.

Now that would definitely account for the vacuum pressure, but would it also have an affect on my AFR? I have read about people having slight leaks but it seems to me I am over looking something major.

I could just be paranoid though..

[9!'clipseDOHC]

Originally Posted by doober75

Yes it is just sitting at zero, and is an aftermarket gauge. I forget the brand off hand to be honest.

Now that would definitely account for the vacuum pressure, but would it also have an affect on my AFR? I have read about people having slight leaks but it seems to me I am over looking something major.

I could just be paranoid though..

Yes, it will effect your AFR, air is escaping but the fuel is still being injected. You will run richer.

[doober75]

Originally Posted by 9!'clipseDOHC

Yes, it will effect your AFR, air is escaping but the fuel is still being injected. You will run richer.

And that is exactly what is happening according to my gauge too. Thanks for the help.

Anything to be careful about when pressurizing my intake? Only pressurize to below my max boost level also correct?

[9!'clipseDOHC]

Originally Posted by doober75

And that is exactly what is happening according to my gauge too. Thanks for the help.

Anything to be careful about when pressurizing my intake? Only pressurize to below my max boost level also correct?

You are welcome!

If you have never done a BLT before I would recommend starting at the throttle body inlet so as to narrow down your search. Once you get that section to hold your desired boost, then move back to the turbo inlet and pressurize the entire system.

I would recommend testing to at least your max boost level if not above. For instance, I'm boosting 20 psi, so I BLT to 25 psi.

Hope this helps!

[Absit]

Originally Posted by doober75

I see that at idle I should be around 19 psi.

Quick correction that no one else pointed out, that's 19 inHg, not 19 PSI. 19 PSI would be around 38.684 inHg.

[doober75]

Originally Posted by Absit

Quick correction that no one else pointed out, that's 19 inHg, not 19 PSI. 19 PSI would be around 38.684 inHg.

Ahh sorry about that, rookie mistake on my part.

[delta448]

Just want to add one thing...

If your boost gauge is actually properly installed by being tapping into the intake manifold, it's not just a blown coupler. No vacuum there would be more likely a symptom of a tooth skipped by the t-belt. In the least, the problem would be someplace in the intake tract between the throttle plate and crank to intake cam alignment. Vacuum is created by the descending piston drawing air through the intake valve with the closed throttle plate limiting the air mass inside the volume of the manifold. When your engine sucks the air out of the manifold faster than the throttle will allow the air to flow into the manifold, you get vacuum.

[doober75]

Originally Posted by delta448

Just want to add one thing...

If your boost gauge is actually properly installed by being tapping into the intake manifold, it's not just a blown coupler. No vacuum there would be more likely a symptom of a tooth skipped by the t-belt. In the least, the problem would be someplace in the intake tract between the throttle plate and crank to intake cam alignment. Vacuum is created by the descending piston drawing air through the intake valve with the closed throttle plate limiting the air mass inside the volume of the manifold. When your engine sucks the air out of the manifold faster than the throttle will allow the air to flow into the manifold, you get vacuum.

It is properly installed. What would be the best way to diagnose a skipped tooth? Any advice on that? I just recently got home for the week and am about ready to start looking into this more. I have not order the boost leak test yet either. Would you still recommend that or what would you personally do?

[delta448]

I would pull off the UICP and pressure test the intake manifold with a boost leak tester attached right to the throttle body.

Remember, hold the throttle wide open during the test and be sure to align the crank and secure it with something so that none of the cylinders have both the intake valves and exhaust valves open during the test.

If that holds well, then the problem is likely with the camshafts alignment to each other and the crank.

[doober75]

Originally Posted by delta448

I would pull off the UICP and pressure test the intake manifold with a boost leak tester attached right to the throttle body.

Remember, hold the throttle wide open during the test and be sure to align the crank and secure it with something so that none of the cylinders have both the intake valves and exhaust valves open during the test.

If that holds well, then the problem is likely with the camshafts alignment to each other and the crank.

Thanks for the reply. I guess I will get myself a test and do what I feel comfortable on my own and hope for the best.

Now I see a couple of you guys are in the state of PA, would you be able to recommend any good shops or places you would trust with your dsm's? Most of the shops in my area don't have much experience in the field.

[delta448]

I almost never take my car to a shop. State inspections and when something needs welding are about the only times (I only have limited access to a stick welder). Honestly, for the rate that the decent shops around here charge for labor, I'll gladly pay myself to fix my junk.

If I don't know how to do something, I research until I understand it. This site is a wealth of info and if you aren't able to find an answer by searching there are many experienced members willing to reply to a new thread.

Get a Factory Service Manual and some basic hand tools and you can fix 90% of the problems you'll have with the car on your own.

[steve]

I'm expecting that perhaps the hose to the boost gauge popped off.

[suprasalas]

did u find out what it was?? i have the same problem with my 2g 7 bolt