Why I do the things I do... I just completed my CA smog inspection today and passed with flying colors. Got home and decided to investigate the minor surge in idle and while at speed with a neutral throttle. I have a 91 Z28 350 TPI stock motor with MSD 6A box. Tore aprart the thottle body and cleaned the IAC valve. Test drive had the same minor surge. Started looking for a vacuum leak. While at idle, it is creating about 15-18 inches of vacuum. Can't find anywhere that says that is good or bad. Anyhow, got out my handy vacuume pump (hand operated) and commenced to checking lines for leaks. Checked the HVAC/vacuum tank/cruise control lines. Took a while to get a vacuum, but it held. Checked the brake booser line and brake booster. Also good. Checked the line from the bottom of the throttle body and it held. Also heard the EGR opening and closing. Checked the MAP line. Everything I checked seemed OK except the heater diverter valve. Couldn't keep a vacuum on it. When I hooked everything back up, it ran like crap and the exhaust was so rich it burnt my eyes. What did I break by putting it under a vacuum. I never went over 15 inches of vacuum on any test and usually stayed at 10 inches. One thing I did notice that seemed strange. After shutting the engine off, I removed the vacuum line off the cruise control. It still had quite a bit of vacuum left in it. Is that normal? I haven't had it running for more than a couple minutes, so no codes have been thrown yet. Would rather try to fix it before running it too much. Any ideas would be greatly appreciated.

If you removed a vacuum line at the cruise servo, you may have pulled the source line. That is connected to the vacuum reservoir, and it would take a while for that to equalize through the small hose. That could also be part of the reason why it took some time to build vacuum in the HVAC system.

I know you passed the emissions inspection "with flying colors" but what colors were they (i.e., what were the O², CO², HC, and NOX numbers)?

Vader,

I figured the reservoir would increase the time it took to build a vacuum. I retested after it started running like crap and the reservoir, cruise control and HVAC lines are still good. Here are the measured numbers and max allowed by CA smog in Ventura county.

At 15 MPH and 1352 RPM:

Measured
%CO2 Measured 11.5
%O2 Measured 5.1
HC Measured 30 Max Allowed 108
CO (%) Measured .04 Max Allowed .71
NO Measured 238 Max Allowed 767

At 25 MPH and 1298 RPM:

Measured Max Allowed
%CO2 Measured 11.5
%O2 Measured 4.8
HC Measured 30 Max Allowed 83
CO (%) Measured .09 Max Allowed .58
NO Measured 199 Max Allowed 706

I also forget to mention that if I disconnect the MAP, the idle smooths out. My gut tells me I killed the MAP sensor. If it's dead and sendig a low vacuum condition to the ECM, it will force fuel to be dumped into the cylinders. Any concurrence before I shell out the money for a new one???

Mike

Replaced the MAP. No joy. I am only getting 8-10 inches of vacuum at idle and only goes up to about 15-17 when I apply throttle (Used to get 15-18 at idle). Replaced all the vacuume lines. No joy.

Thought I would go back and look at the work I had completed on the throttle body before the good test drive. Pulled it back off. Pulled the IAC, reinstalled and checked that it closed before reassembling to the throttle body. Checked sat. Reassembled and still idles rough.

When it is running, the intake has what I consider to be a louder than normal sucking sound. I reinstalled the air cleaner cover and boot and it still sounded loud. Rechecked all the vacuum connections on the plenum: MAP, Brake Booster, PCV, fuel regulator, and EGR (throttle body to EGR solenoid). All are good.

Checked fuel pressure on the rail and it is 42 psi. Think that is correct but could be wrong.

I don't have a scanner and don't know anyone that has one, so I can't check numbers.

Mike

Well, I drove it. Then, against my better judgement, I drove it WOT for about 10 seconds. Then at the next stop sign, it ran fine. Back to the original minor idle hunt that started this whole fiasco. Exhaust smells fine. I really don't understand this one. Now I can continue with my previous surging idle problem. Just now I don't have to worry about it being a vacuum leak, the MAP, or the O2 sensor. Still not sure about the IAC as it had some pitting around where the pintle seats in the throttle body. Is that justification for a new one?

Mike

Sounds like the EGR valve is having issues.

Causing the idle surge, the crappy idle that was corrected or both?

I had the fuel injectors cleaned with the BG system. Vast improvement over what it was. Still has the surge and a slightly rough idle. Put a spark checker inline with all the spark plugs and they all tested sat. Pulled a couple plugs and they were black from the extremely rich condiditon. I'll change those this weekend. Everything else seems fine with the ignition. I am getting an EGR valve and solenoid to replace this weekend also. I understand there are positive and negative pressure solenoids. Is there any way to tell which one I have before I pull the plenum? I would like to have all the parts before I start. Thanks in advance for the assist.

Mike

EGR valves operate with either positive, or negative exhaust back pressure; the part number should end with a P, or an N.
The solenoid controls the EGR valve with manifold vacuum which is negative pressure, positive pressure would not allow the valve to open.

I would assume either one would workon the car regargless of which one was originally installed. I am trying to get all the parts ready in advance since none of the stores around here cary the EGR valve in stock. I would have to rip it apart and have the car inop for a couple days waiting for the EGR valve to arrive.

Mike

EGR valves are calibrated for specific engine applications. The wrong valve may flow too much or not enough exhaust gases and cause emission, driveability, and detonation/pinging problems.

I called the dealer and gave him the last eight numbers of my VIN. Only one EGR valve part number was listed. Guess I got lucky, but at least I didn't have to tear it apart to find it.

Guess I can close this out. I replaced the injectors and EGR valve along with the intake manifold gaskets and plenum and runner gaskets. Runs like a brand new car. Rock solid idle, no misses and also resolved a vibration problem at speed. Thanks to all that replied.

I did test the EGR valve I replaced and it was good. Therefore, I assume the injectrors were the culprit to most of my problems.