This was a repost of a thread in General Tech, perhaps you guys can help!

Original Thread:
https://www.thirdgen.org/forums/tech...-firebird.html

I have a 1988 Firebird, 2.8 V6 MPFI.

The car has a hard time maintaining idle. It is acting like there isn't enough air for it to run. I am fairly confident that this is a computer/sensor issue, because when I reset the timing to manufacturer specs, I disconnect the ECM. When I do that, the engine runs normally. Once it's reconnected though, it goes back to running like crap.

Looking through WinALDL, it shows raw MAP sensor as zero. I am not sure what to think here. It also shows the battery voltage as 5 volts, so i'm not sure if I can believe it. The log is attached in the original post.

I checked via continuity test the two 20 amp fuses located on the passenger side next to the charcoal canister. Both registered good. I probed the wires going into the MAF sensor with the engine idling (rough), and here's what I got:

black wire - 0v
black/white wire - 2.3v - 2.5v (varying)
Red(ish) wire - 13v

A few hours later, I turned the car key to the "on" position but left the engine off, and measured the MAF voltage... I got 2.5v. According to the Haynes, the MAF puts out a voltage of 0.5v to 5v depending on the amount of flow across the sensing unit. This leads me to believe that the MAF is really bad (or I got air flow with the engine off

What do you guys think? I'm assuming that at idle (or off), the MAF should either give a voltage of 5v or 0.5v, and 2.5v is way off. Thanks!

~Toast

 

Your MAF output is a variable frequency, not a variable voltage.

As for the SA timing, once the EST BYPASS is re-connecteed and the engine restarted, the timing should be about 22 BTDC. Check it with a light to be sure it does that.

RBob.

 

RRob:

Your suggestion prompted me to do another WinALDL logging run. In this run, I realized the reason WinALDL reported 0 for the MAF is that the car wasn't moving. So I took it for a drive while logging and I got (what look to be) sane values.

But on this log which i've posted, the car actually died on me. I've attached the log.

Afterwards, with the car not running, I tested the TPS voltage, and at idle it would mostly be around 0.5v however there were definitely a few times when it would get as low as 0.1 or 0.2 volts. Sometimes for a few seconds. Turning the throttle up all the way set the TPS to 4.5v.

I also noticed the MAT sensor was particularly high just before the car died (381), and it varies a lot.

 

Ok checked MAT sensor resistance, and it was within normal values for the ambient temperature. I even put in a 5k resistor in the MAT plug, and the car continued to idle badly. I will not attempt to clean out the intake with FI intake cleaner, and readjust the IAC.

 

Readjusted the IAC. I now have some more questions:

In the IAC Tech Article, it said to set the idle to 450 RPM. I was only able to get it to 550 rpm before the car was seriously starting to die. I question whether 450 RPM is the true rotational speed at idle for a 2.8 V6. Or maybe my motor is just in bad shape?

Second, I found a site that stated GM Delco MAF will output a constant voltage of 2.5 volts (which mine does). In the same book, it says that older AC Delco MAF's as found on 2.8 V6 are frequency type, varying from 30 hz at idle to 75 hz at 3500 rpm. Are GM Delco MAF's and AC Delco MAF's really the same? I'm wondering if I have an incorrect MAF in my car...

 

I think you are supposed to have a constant 2.5 volts at 30Hz (idle) so both statements are correct, they just each left out the other half.

 

So we are having the same type of problem with our 2.8 86 Camaro. If we have the MAF unplugged the car runs fine. As soon as we plug it in though the car can hardly idle and will run very badly and back fires/stalls. We have tried a new MAF, which didn't make any difference, thinking it may have been a bad MAF we tried a second one, still no luck.
Our "official" camaro shop manual say to check the voltage on the "b" terminal of the wiring harness to the MAF with the key to on position. The book says if it less than 1 volt, it is a faulty ECM connector terminal B6 or a bad ECM. Our reading on this terminal was .07V...
We have also put a new MAF relay which didn't help.
We are going to the store now to see how much the ECM terminal B6 and ECM will cost...
We are going to try the cheaper part first to see if that was the problem.
We will keep you posted...

 

Alright so no one knows what this B6 terminal is. We talked to Kragen and looked all over the internet...We also couldn't find any type of ECM wiring harness...Any ideas?

 

OK, I have some real data, and a possible conclusion!!!

I went and purchased a voltmeter at Fry's that measures frequency (hz) and took some measurements. Without flooding the forum with my handwritten notes, here we go:

Half the time when I turn the car on (not started), the voltage at the MAF signal wire varies from 0.6 -> 3.5 V. This happens around 2.77 hz. If I start the car with the MAF acting this way, the frequency output of the MAF does not change (2.77-2.79 hz), even if I rev it up from 900 rpm to 2000 rpm. If I cover up the loose fitting intake to the MAF with my hand, the car will not have any noticeable effect (will keep running). I call this BAD MAF mode.

The other half of the time, I'll turn the car on, (not started), the voltage at the MAF signal wire holds constant at 2.6 V (not really constant, just that the VOM can't register the voltage change that fast). Frequency registers between 10.2 -> 10.5 hz (as expected). If I start the car the frequency output of the MAF will rise to 40.7 hz at 900 rpm (idle), then to 50.2 hz at 2000 rpm, then back to 40.9 hz at 900 rpm (idle). If I cover up the loose fitting intake to the MAF with my hand, the car will suffocate and die. I call this NORMAL MAF mode. RPM Measurements were taken at the tach.

My hypothesis: If the car starts in BAD MAF mode, the computer ignores the MAF and uses preset values, and opens up the IAC, allowing air to flow even if I cover up the air intake (with my hand) at the MAF. If the car starts in NORMAL MAF mode, the computer thinks the MAF is sane, and will adjust the IAC to restrict airflow at idle.

Since 40 hz at idle sounds high (supposedly normal idle flow is 30 hz), the computer should close up the IAC much more than normal at idle to restrict the non-existent high air flow. This I think is what is causing my rough idle. If I cover up the intake with my hand, the car dies because it's already restricting airflow at the IAC.

<thirdgengirl>

If the car is running badly only when the MAF is plugged in, i'm thinking that your MAF really is good, but your IAC may be bad. If you open up the throttle with the idle set screw, I'd bet it would run a little better. If you're fuel injected, the idle set screw is covered up by a piece of metal (to avoid tampering, as per GM). You'll have to punch it out with a small screwdriver, or metal punch to get it out. I would recommend also measuring the signal wire of your MAF in terms of frequency and see if you get sane values.

</thirdgengirl>

I'm going to remove the MAF from the car and try and reproduce the results with a 12v battery on my desk. I believe if I just keep switching the 12v to the MAF, I should be able to get it to BAD MAF mode half of the time. If so, then I think it needs to be replaced.

What do you guys think?

 

It does sound like you have a bad MAF. You can go to Kragen and buy a remanufactured one for not too much.
We ordered a new ECM, we should have it tomorrow...I will let you know what happens...

 

Hi I had the same problem on my 86 camaro 2.8 and codes kept refering to the maf sensor. Changed it twice and cleaned the IAC a coulpa times and then tried a new auto repair shop and they found loose screws on some where on the throttle body and problem has been solved. I used to have to finnese the pedal to keep it running at idle when engine normally run after 10 minutes and even then it would usually stall and need a restart, but now I have a happy starter and great idle again. hope this will help

 

So I have no idea what is wrong with my car... We got the new ECM and installed it and still no luck. The car will not turn on when the MAF is plugged in, but when we unplug it, it runs okay. I just have no more ideas of what it could be...We tightened all of the screws we could find on the throttle body and still no luck. Any other ideas?

 

one thing a lot of people forget to do is checking the grounds (especially the chasis grounds) a bad ground can cause quite a lot of "weird" things to happen.
IE. I once installed a new Audio HU into my 83 (before it was wrecked) the ground i had choses was not the best in the world, which in turn caused my dashlights to not light up when the headlights were on OR caused my dashlights to come on but not my head lights, took a while to figure out that one.

 

thirdgengirl - have you tried putting a small piece of cardboard or something to force the throttle plate to be more open than it normally is? If this allows your car to start, I would seriously suspect the IAC. Unless you've ruled it out already...

~Toast

 

For everyone's information: the idle speed of your car should be written down on your VECI label. In hindsight, I felt kind of dumb because I should have looked there first. So for people who don't believe the 450 RPM engine idle rating in the IAC/TPS tech article, check it against your VECI.

 

Okay so I think that I have tried all of the possibilities posted here...Still no new infomation. So let me go over again what my problems are and what I have done...
The Problem:
When we first recieved our car a few months ago it ran very poorly and would almost die at idle. We discovered that if we unplugged the MAF sensor the car would run very well. Over the past few months though, running the car on weekends with the MAF unplugged, the car's performance has taken a nose dive. With the MAF plugged in the car won't even start now and with it unplugged it runs as terrible as it did when we first got it and had the MAF plugged in.
What we have done:
New MAF sensor
MAF sensor relay
Air filter
Oil change/oil filter
Spark plugs
Spark plug wires
Fuel filter
fuel pump
Checked the IAC
Tighted all bolts on throttle body
O2 sensor
NEW ECM!
We have also put new fuel and used a fuel system cleaner.

We have taken it to two different mechanics and they both told us that we need a new fuel pump because they tested the fuel pressure( it is at 20 and it is supposed to be at 40). We told them that we put a new pump, after which they suggested that we may have put in the wrong fuel pump. After double checking with the store we purchased it from, we know that we have the right pump.
I suppose the only things we can do are to check all vacuum lines, assume that we purchased a defective fuel pump and install a new one, check the fuel system for clogged or defective hoses.
Any ideas guys?

 

Disconnecting the MAF puts the computer into limp home mode... It basically bypasses all the sensors and controls the bare minimum necessary to get the car to a shop.

If you've got 20psi on a MPFI car you've got a bad pump, bad regulator, or severely leaking injectors. Could also be a wiring problem between the fuel pump relay and the pump itself, like a bad fuel pump ground, causing a voltage drop preventing the pump from reaching maximum efficiency. Even with really low fuel pressure, it'd probably still run, just not very well.

Also it's not unusual to get a part from the parts store in the wrong box. The people in the manufacturing facilities don't always care how the product works on your end, and parts store monkeys have a bad habit of taking things out of boxes and switching them around while letting customers look at the parts, or just to **** off customers. I wouldn't rule it out just because the box, receipt, parts monkey, tells you it's the right part. Verify the part numbers online yourself at www.cskauto.com or www.autozone.com etc.

 

Did she ever solve her problem ??????????????????

 

Not yet...

 

I have the same problem with my 87 firebird, I'm told by LOTS of mechanics that these sensors are faulty. Does anyone here know how to bypass them? I heard there's a way, but I don't know how it's done. I'm returning my NEW MAF sensor today to exchange for yet another one. The new one was defective again. Anyone who knows how to bypass PLEASE let me know.

..bert

 

When getting a new MAF I was told that you have to unhook the battery whe nyou unplug or plug them in. If not it can fry and they will not warrenty it.

 

OOPS...there's egg on my face. I didn't know that. Oh well, I'm getting a new one anyway, but I will be careful with the next one. Other than that did you hear about the bypassing of those sensors. The vehicle will probably run better without all those darned sensors & garbage. Anyway, I hope not to have to bypass it, but if all else fails, then I will want to do the bypass thing.

...bert

 

have you changed out the fuel pressure regulator in the fuel rail if it is torn it will not build up pressure