chevy 350

[Daniel]

I have a 1987 iroc with a 350 multiport injection about 150,000 miles. I have changed pretty much every sensor on the engine, spark plugs, wires, cap, rotor, alternator, battery, fuel filter, pretty much everything you can think to change I have done it over the course of the last year.
Check engine light came on today, (after a 2 hour trip) and it idled a little uneven (very slight.) So I hooked it up to my computer and it gave me a code 34 (maf low voltage/high vacuum.) The maf is brand new and the connector looks clean. I never cleared the codes from before though, so that point may be moot.
While it was idling uneven I checked (on my computer) the tps. It was at .54 volts which is perfect. Also air idle valve is good. But it did keep saying that it was running lean and then rich and back and forth. Used my fuel pressure gauge and pressure is good at 43 psi steady(55 psi when I unplug fuel pressure regulator.)
It’s really not affecting anything, I just wanted to pick your brains.
Oh and I’m guessing the maf connector is supposed to have around 4 volts coming out of it into the maf right? Cause it doesn’t say in my manual but I do know it operates within a 4 volt window.

Topic

[Mike]

does it run okay? If so, clear the codes and see if it comes back.

[Daniel]

runs good other than slightly uneven idle and lost some power for a split second once during 2 hour drive.

[college man]

here is a link for some help.

http://www.iroczone.com/2009/10/1982-1992-ecm-trouble-codes-for-camaro-firebird/

[Daniel]

Thanks for that link, it’s more informative than the one I was looking at. I think that code is from before I changed the maf though. One thing I do need to look into soon is: when I had the computer hooked up to the car it said that (at idle) it had 20 degrees of advance, and at one point it jumped up to 34 degrees. Between physical testing and computer diagnostics nothing else came up out of spec. Could the ignition module have something to do with this even though I never got a code for it?

[college man]

actually according to the link I sent you.The voltage
at idle should be 0.5 volts at idle and 4.7 volts at wot
your at 4.0 volts if I read your thread correctly. according
to one of the symptoms its reading excessive air flow.Try reading
the voltage at the ecm and the maf.

[Daniel]

I never actually read voltage at maf, I just was kind of assuming it should be around 4 volts. I was going over the car but the temp outside dropped and I haven’t done anything else with it yet. I figured I would try to formulate a strategy for when I had a chance to go back out there.

[college man]

keep us posted.

[EricTheCarGuy]

An often overlooked problem is ‘pirate air’. This could be a leak in the hose from the MAF to the throttle body. This would be unmetered air and would cause the MAF readings to be off and possibly set a CEL. In addition vacuum leaks can do the same thing so be sure to check for those as well.

Solving Performance Problems 1

Solving Performance Problems 2

Re Performance Problems

[Daniel]

Thanks man I’ll check out those videos tonight. Last night I found that I was loosing vacuum through the climate control system. So I corked it. I haven’t run any codes on it but the idle seems to have smoothed out a bit. So now I need to figure out the ignition timing, check the voltage at the maf, and resolve the climate control vacuum loss. I’ve givin this car more attention than my girlfriend.

[EricTheCarGuy]

If you found vacuum leaks this is something that can offset the MAF readings so keep that in mind when you do your testing.

[Author]

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