loflyin

1986 stock L98: Code 21, 33 and 36. Broken wire in TPS connector, replaced the connector and then showed only Code 33. Completed flowchart for Code 33 from FSM. All signals checked out and the final evil box at the bottom of the page says bad MAF or ECM...... So, my question is: how do I know which it is without swapping out parts that I do not have and do not want to buy? The ECM feed shows 4.99v, but this is between the 4 to 6v specified for the test. How can I tell what signal is going to the ECM. I have cleaned the MAF and the connector, with no difference noted. Car sat for a couple years and was run every two months or so. Didn't seem to have a problem until I wanted to start driving it daily again.

JrRifleCoach

Can you describe how the engine runs?

Have you cleared the codes and taken it for ride to observe if the same codes have returned?

Cliff Harris

The best way to troubleshoot this is with either a hand held scanner or a laptop, cable and scanning software. That will tell you what the ECM thinks it's seeing from the MAF.

You can also hook up a digital voltmeter to the MAF output signal and see what is happening. The easiest way to do this is to shove a hat pin or small paperclip into the back (wire side) of the MAF connector and attach the voltmeter leads to it somehow.

I bought a package of these pins at Walmart. They are not as strong as a real hat pin, which is why this one bent when I shoved it into the connector:

loflyin

I have a laptop, but no connector or program. With a voltmeter, what connections should I make? Terminal C (green wire/CKT 998) is the ECM signal, correct? Pin in this terminal, connect positive of VM and ground other? Or is this the signal from the ECM? Hate to sound pathetic, but don't want to burn a quad driver that was good. And yes, I clear codes every time that I do anything that could possibly change them.

loflyin

Oh, as far as how it runs: Starts fine, cold or warm. Idles up a little as it warms and then slows. Hesitates on takeoff, serious lag and loss of power if you give it much gas. Starts missing pretty bad the longer it runs and after 8 - 10 minutes, it may shut off when you let off the gas. In the past month, replaced FPR with stock diaphragm, replaced Orings, fuel pump. Notice a cracked insulator in the top of the EGR diagnostic wire today, any bearing on current problem.

jesredvette

Well, I don't have the knowledge to respond in detail, but.....
I had the exact same symptoms with my '88 a couple of years ago.
Installed a remanufactured MAF from Autozone and all symptoms were gone. Car has run fine since.

Cliff Harris

That sounds like a bad spark plug wire. Check for arcs in a dark garage.

Cliff Harris

The green wire is the signal from the MAF to the ECM. I didn't want to specify a color or pin letter because there are two different versions of the MAF and they have different connectors.

Connect one lead of your meter to the green wire and ground the other lead. It doesn't matter which lead you use (the electrons don't care what color the insulation is). If they're backwards the meter will read minus, but who cares? It's the number that is important. It should be low at idle and go up as the RPMs increase. The range is close to zero at idle and up to 5 volts at high air flow. You can see this while sitting in the garage. You don't need to drive around with the meter attached.

loflyin

Thanks for checking back, Cliff. What would make you think that I would do something like ride around with a VM attached under the hood? You been watching me? Seriously, I do something stupid every day...I just don't always know what it was! Couldn't work on it tonight- two rounds of thunderstorms. Will check this tomorrow. BTW, noticed a lot of the help you have been offering folks on this forum. Good man.

loflyin

OK, finally got to check this. With just ignition on, MAF reading is 2.64V. On cold startup, it only reads 1.7 and after running for 3-5 minutes, it reads 0.9V. After 10 minutes, it dropped to 0.58v - 0.7. It threw a Code 33 after running only a minute or two. Shut it off, cleared codes and repeated this with a different meter (just in case). Readings were essentially the same. However, there were no codes the second time. Running a little better, but still has a slight miss. Before cleaning the MAF, voltage using the FSM flowchart was 4.99 and now 2.64 after cleaning. Did it take a few days to realize that it was clean? Why did it throw a code 33 the first time tonight when the voltage was only 2.64 and then dropped? Don't notice the strong fuel odor now. Still has slight hesitation when you rev it up. Any thoughts on this?

Cliff Harris

When my car is idling the MAF reads a few tenths of a volt.

The ECM supplies 5 volts as a reference to the MAF and the MAF pulls that voltage down to whatever it's trying to communicate to the ECM.

Strong fuel smell is a prime symptom of a ruptured FPR (Fuel Pressure Regulator) diaphragm. To check for this, disconnect the vacuum line from the FPR and turn on the ignition (don't start the engine). Check for fuel coming out of the vacuum port on the FPR. If this is your problem, fix it right away. It caused a fire in my car.

loflyin

No leaks at FPR. No codes again today. It appears that cleaning the MAF did help. Tinkered with timing. Went for a ride and no more dying down when stopping. Good deal more power on takeoff. This weekend I will give her a good onceover to include all vacuum lines (again), new plugs and nail the timing. Thank you for the help and rest assured, I'll call again if efforts this weekend are not fruitful.

Cliff Harris

I suggest a minimum idle adjustment:

http://forums.corvetteforum.com/1570563689-post1.html

There is some controversy on whether or not you need to disconnect the EST connector. Some years say to disconnect it in the FSM and other years don't mention it at all. There are two versions of the procedure written by Lars. In the B version he quotes the FSM where it says to disconnect the EST connector, but he doesn't say to disconnect it in his procedure. It's much easier to keep the engine running if you leave it connected.