Could it be the MAP sensor?

 

OK, I'm assuming your fuel pressure is good (38-42) when it's acting up, right? I want to fully rule this out before moving on.

If it is, lets dig in further. Check to see if you have any stored DTC's (diagnostic trouble codes) in the computer. Here's how.

With the key off, use a paper clip or short piece of wire and jump terminals A & B together on the ALDL. This puts the computer in diagnostic mode.

Turn the key on and watch the check engine light. It will begin flashing. It will flash each stored code 3 times. It will start with a "12" code, which indicates the ECM is functional. A 12 code is "flash, pause, flash flash" followed by a long pause. This will occur three times, always at the beginning and again at the end. After 3 "12" codes, the computer will give you any other fail codes in sets of three. When it's done, it will go back to the 12 code again to indicate no further stored codes.

As an example: if you had a "34" code (MAF) stored, the computer would do this. At key on....

12 code three times (flash, pause, flash flash)
Long Pause
34 code three times (flash flash flash, pause, flash flash flash flash)
Long Pause
12 code three times (flash, pause, flash flash)

......repeat until you shut the key off or pull out the jumper wire.

If your fuel system is fully functional...that it, you have 38-42 psi when the failure is occuring, then we can rule it out and look at emissions. Once we rule out emissions, we move on to the ignition system. Remember the three things they need to run....compression, fuel, and spark.

Post which codes, if any, you have stored, and we'll go from there.

 

It could, but let's troubleshoot before we condemn it to death and spend $$$ and time on it, only to have the problem resurface. Pull the trouble codes, and see what rears its ugly head.

Oh....one thing to check while you're pulling codes. You symptoms are also point to a bad ignition module....kinda like this guy

Ignition module thread

An ignition module is easy to test if you have a tester, and if not, Autozone will test it for you. Ensure that it has a light coating of module (dielectric) grease under it when reinstalling. Have it tested, don't just replace it. They don't fail as often as they get blamed for.

Here's my famous quote that I love to beat into the ground.


We just need to find out which condition above isn't being met when it won't run, and then narrow down why. Keep your chin up, we'll hit the problem.

 

Ok, will try that and let you know. Thanks buddy!

It's frustrating, cuz when i bought the car the guy gave me a bunch of new parts he was planning to replace, and I'm trying to figure them all out....a few of them have me stumped. http://www.flickr.com/photos/8061286@N07/

 

A & B are the two slots on the far right, correct?

 

This should help.

 

dude you rock, i'm getting right on it....

 

it is straight up 12 codes.....1 flash, pause, 2 flash......repeats that forever.

 

OK, good (38-42 psi) fuel pressure, no codes, idles and revs fine with no load on the engine, stumbles and backfires through intake under load.

Let's look at the ignition system. You stated you put new plugs in it recently. Did the problem start before you did this or after? You're 100% sure the wires are on the correct plugs and distributor towers, right? Run the firing order and verify....there's no shame in crossing one...most of us have done it before, and the rest have done it and just won't admit it.

What brand of plugs did you use, and what did you gap the new plugs at? Let's take a look at the cap and rotor....what condition are they in? How do the wires look? Next let's check your base timing. With the ECM wire disconnected, it should be set around 6 degrees BTDC. You can get away with up to about 10 degrees usually before it starts to detonate. (Note...after disconnecting ECM wire to check timing, disconnect battery to clear codes in ECM).

If everything checks out there, test the ignition module (buy a tester or have AutoZone do it). Ensure that there is a light coating of dielectric grease under it. The grease acts as a heat sink and is critical for correct module operation. I'm suspecting a module failure may be the culprit in your case, but let's test it before buying a new one.

Next, let's check it for vacuum leaks. If you don't already have one, pick up a vacuum gauge, attach it to any manifold vacuum port, and post how many inches of vacuum you have at idle, and if the needle is steady or jumpy. Test the EGR valve operation while you're at it....the valve may be hung. An inexpensive vacuum pump will do this.

That should keep you busy for a spell. Post if you need any help on testing procedures. I highly recommend you pick up a Factory Service Manual for your car. It's the best $157 you will ever spend on it. The FSM has pulled me out of more binds than I can count, and has paid for itself several times over.

Have fun wrenching

 

Thanks buddy.

The problems didn't start like this until after i replaced the fuel pump, so not sure there. It was firing and running fine before the fuel pump replacement and after the spark plug change, so that should be good.

I also replaced my egr after doing the plugs, so that is new and good.

I will do the vaccuum test and let you know what pops up.

 

Sorry, out of town again, this time dreary Iowa.

I am committed to getting my baby back on the street this weekend.

Vaccuum is good, no leaks.

However, now it won't crank over.....ran the pressure gauge and it is now showing zero pressure. Any ideas? Fuel pump relay?

 

Won't crank or won't start?

If you have zero fuel pressure, run 12 volts to terminal G on the ALDL and tell me if you have pressure then.

If the relay is bad, it will still start, it will just take a long time to do so. There is a built in bypass....the oil pressure switch. If the relay has failed, the oil pressure switch provides an alternate method of energizing the pump once oil pressure reaches about 4 psi or so.

Running 12v to terminal G takes all of that out of the equation. I'm starting to suspect you have some wiring issues to the fuel pump circuit. You have checked the fuse for the fuel pump, correct?

 

Terminal G is the diagnostic connector for the CCM on a '90. I'm not sure you want to connect 12V to it.

I've been digging through my FSM and find several references to the 'Fuel Pump Test Terminal' but the location is not specified.

In the Electrical supplement, there is a Fuel Pump Prime Connector shown in the engine wiring harness, but no exact location is given.

Not a lot of help, but possibly cooking the CCM would not be a good thing.

 

Didn't want to jinx it again, sorry took so long, wanted to make sure the solution held...but here's what happened. Ok, i went back to square one, wasn't getting any pressure, 1-2lbs only, but fuel was flowing, just without pressure. Went thru many steps but finally had a buddy come over and go thru it with me, needed a fresh pair of eyes. Though he was unfamiliar with the fuel pulsator he suggested we try it with the pulsator pulled down over the fuel pump line )previous to that it had ridden up to the top of the fuel pump assembly), so we slid it down over the fuel pump connector and low and behold my baby is running again. Something about the pulsator helping bump up the pressure, not sure how it works but that was the key to building the pressure in the lines. Now i can cruise once again, can't tell you how nice it was to have my baby rolling again.

 

Congrats on successfully troubleshooting and repairing the problem. Sounds like you found the source and corrected it.

Incorrect. Terminal B is the diagnostic connector. You would ground it to Terminal A to run diagnostics. Terminal G energizes the fuel pump. '84 -'91, all the same.

Then you're not looking hard enough. Try looking on the ALDL. Specifically Terminal G.

It's hidden under a small "G" on the ALDL. How sneaky of them.

Correct.

No, it wouldn't. But 12V to terminal G ain't gonna do that. Thanks for playing.

 

Incidentally....in case you want to read the wiring schematic, the fuel pump circuit is #490 (fuel, pump prime, feed). If you look in your service manual you should see that terminal G corresponds to.....you guessed it, circuit 490. The diagram for 88-91 lists it as a red wire.