Little background on the car. It's a 91 Z28 with a 350, headers, exhuast, !AIR, !smog, and a t56 swap.

The problem: After doing the swap, I noticed my car was backfiring on deceleration quite a bit. I traced that down to a lean condition caused by burnt header gaskets. Here recently, I have changed all the plugs, wires, and put in new header gaskets. My problem is that my car is missing and throwing a code 34 (MAP circuit or sensor too low) as well as still backfiring like crazy.

With the code issue, I have measured voltage with a scan tool and noticed that when the throttle is held open, the voltage only temporarily increases then drops down to around .85-1.2V. Idle voltage seems right on at 1.45-1.56V. I checked at the sensor for the 5V reference from the computer. It was good. Checked the ground in the circuit, it was also good. I then backprobed the MAP signal wire at the sensor and the voltage there matched the voltage I was seeing on the scan tool. Welp, tried a new sensor, same symptoms. With the key on, engine off, the voltage measured with the scan tool was right at 5V. Are there any other ways to test why the voltage coming from the MAP won't stay steady in the 2.5-3V range when the throttle is held open?

This is seeming to cause my other problem which is the miss once the car goes into closed loop. The o2 sensor is brand new, as are the plugs. Pulled the plugs today to check them out and they were all as black as night which is an idication of a rich condition, correct? Well, says the scan tool, the o2 is reading that the exhuast is lean, even with black plugs? I'm thinking there is something causing a lean condition making the MAP overcompensate and dump more fuel into the engine. There are no vaccum leaks anywhere, all connectors look fine, no exhuast leaks anywhere, so I'm just rather stumped. If anyone has had similiar experiences with this type of issue and could point me in the right direction to fixing it, I'd greatly appreciate it.

"!AIR !smog"

Exactly what components did you remove? What cam is installed?

The lower MAP voltage indicates a fairly decent / high idle vacuum. Is there a PCV valve installed? The black plugs indicate a rich condition.

You can test the MAP output voltage by connecting a vacuum pump. With the ignition on, the MAP output should be close to the table vales at a given vacuum applied:

Well the AIR pump and the solenoids above it were removed, and I have no cats. Basically everything with the air system was removed except the wiring.

I'll try to hook up a vacuum pump to the MAP and see what values I come up with. Some other research suggests that the PROM might be an issue due to them going bad or something? Something else to possibly check.


Ah yes, now my car is throwing a code 44(?) "Lean exhuast". That was after the install of a new O2 sensor. I haven't checked the plugs as of yet after cleaning them, but from the backfiring I"m sure they are again pitch black.

Oh yes, stock cam. Only mods to the car are listed above.

Thanks for any further help.

When the air injection system was removed, were all the ports in the manifolds plugs and sealed? Any air leakage into the exhausts will contribute to your problem.

That doesn't really address the '34' code, however.

You indicated that the scan tool measured 5.0V reference, but that is at the ECM connections. What is the ACTUAL voltage at the MAP sensor out on the engine, as tested with a real voltmeter? It could be considerably different than what the ECM and scanner are reporting.

Also, what is the TPS voltage at idle?

Well with the change to hedders, I'm pretty sure I have solved the exhuast leakage issue. I don't seem to have any exhuast leakage anywhere that I can tell. Like I said, new hedder gaskets and I just had new exhuast welded up so it should be air tight. I am only running a bullet muffler off that dumps before the rear axle with a crappy y-pipe at the moment.

I did measure the 5.0V reference at the connector to the MAP sensor itself. I read 5.0V constant with the key on engine off and with key on engine running. Also, the ground in the circuit seems to be working properly as well.

At idle the voltage measured both on the scan tool and at MAP sensor itself (back probed the wires) was around 1.4V-1.56V. The voltage does jump momentarily when the throttle is snapped but then settles back down in the 0.8V-0.95V range, then it will come back up to 1.4V back at idle. Does the same thing when the throttle is held at a given rpm. It spikes momentarily to the voltage is should be putting out, then settles back down to less than 1.0V after a few seconds even with constant throttle. That is basically the part that is stumping me.

TPS voltage seems to be around .54V at idle I believe. It looks about right from what I am seeing. I'll have to go back and actually write down the values to be sure though.

I did notice, while playing with the scan tool again, that I have 0% EGR duty cycle. The EGR does not seem to be activited at all. Would a malfunction in the EGR system cause a false vacuum reading to the MAP sensor and be the root of the improper readings it is putting out?

I still need to get my hands on a vacuum pump to measure the readings the MAP sensor puts out with vacuum.

The EGR solenoid should not get signal from the ECM until the engine is at a moderate RPM, has a relatively low LV8, low TPS signal, low MAP, and the transmission is not in PARK or NEUTRAL. It may also depend on some VSS signal, but I'm not certain of that on S/D systems.

Welp, I have no VSS signal right now since I have yet to get the SGI box to convert the signal to the speedo. The ECM senses that the transmission is always in gear since that is how I have wired it up.

Little update, my car now seems to have idle issues. On relatively cool mornings or evenings the idle will stay at around 1000rpm even after warm and sometimes will stay around 1300rpm at a stop. I'm thinking this is an IAC issue and possibly could be causing my lean condition. I have to further investigate that one.

Other than that though, the MAP issue is still puzzling me. I can definitely feel the lack of power as of late since the car seems to be struggling to get going. It's rather frustrating since this car has plenty of power when it runs right, but it just doesn't