How to diagnose computer failure

[skullandbones]

I am getting close to engine startup so I have a question regarding the EFI computer. I have never had an EFI system so if your computer is not operating properly, how would you know? On a carb system, I would figure out if I didn't have spark or fuel pretty quick. Timing issues would be easy to recognize. I know that the engine can run when the CPU is in "limp" mode. My concern is that I have a '92 model donor and this box has been sitting around (in a safe place) for quite a while. Of course, if it starts up I know it works.

If I hear the fuel pump buzzing when I get ready to start that means at least some of the system is working because the fuel pump hot wire goes thru the computer harness. That's the only thing I can think of that would indicate anything before the start.

Has anyone had a total computer failure and what did you do? Also, how long did it take you to figure out what was wrong? The worse case scenario is that I have to order a new box but the good news is, I could start out with a custom tune for my setup. Thanks in advance, WEK.

[CapeCoralCobra]

I switched from speed density to MAF last year and when I put in a used MAF A9L CPU the engine would run for a few seconds and die. I hooked up an OBD I code reader and it had all kinds of error codes - multiple sensors all out of range. After research & asking questions, I borrowed a good CPU from a fellow forum member - Thanks, Stoked! - and all the error codes disappeared. Verdict: The CPU was toast. I got a reman. A9L from Auto Zone, less than $100 with trading in the old one as a core, and problems were gone - no error codes.

I bought the code reader through Amazon, inexpensive - maybe $30 - and it came with a book that details/defines all the codes. I didn't connect a "check engine" light, so I plug it in once in a while just to make sure everything's OK, real simple, right from the driver's seat. The reader will check codes in three modes, engine off (KOEO), running (KOER), and continuous memory (CM), to analyze specific items. It will tell you if the engine is in the limp mode. You can check for codes with a test light/volt meter/small light bulb & a couple wires - http://www.oldfuelinjection.com/?p=13 - but it was worth the $ for the convenience & book.

Edit: Where the linked site shows the test connections under the hood, in the roadster you can run those connectors so they are under the dash near the steering wheel, hence, from the driver's seat.

HTH

[OCCPete]

I had one die. When I hooked up the code reader and flipped the test switch, nothing happened. The other indication is when you turn the key on, if the fuel pump either doesn't come on at all or it comes on but keeps running.

Pete

[skullandbones]

I have a MAF system from a 92 GT. I got a Summit EFI harness after wrestling with the donor harness for a month and couldn't figure it out to marry it to my kit harness. The MAF and injectors are matched (24 lbs) so that should be OK. I also have a STO and STI that come off the EFI harness behind the dash. From the link you provided, it looks like the code reader would be a good investment. So if this baby doesn't start up and keep running, I guess I will be doing the code diagnostics to see if the CPU is fried.

Curious: if your fuel pump continues to run wouldn't that eventually burn it up? I thought there was some protection device that caused the pump to stop when your line got up to pressure like some sort of feed back device.

Thank you for your posts, WEK.

[skullandbones]

I have a MAF system from a 92 GT. I got a Summit EFI harness after wrestling with the donor harness for a month and couldn't figure it out to marry it to my kit harness. The MAF and injectors are matched (24 lbs) so that should be OK. I also have a STO and STI that come off the EFI harness behind the dash. From the link you provided, it looks like the code reader would be a good investment. So if this baby doesn't start up and keep running, I guess I will be doing the code diagnostics to see if the CPU is fried.

Curious: if your fuel pump continues to run wouldn't that eventually burn it up? I thought there was some protection device that caused the pump to stop when your line got up to pressure like some sort of feed back device.

Thank you for your posts, WEK.

[CapeCoralCobra]

I, too, had no experience with EFI when I started my build. Check out all the other topics on the webite I linked, lots of great information there and where I learned most of what I now know. It's also where I bought wiring/ small parts to switch to MAF. Not sure about the fuel pump. When I turn the key, I can clearly hear the pump for just a second or two and then it stops. EFI Harleys are just like that, and HD says to wait for the sound to stop, then hit start. Recently, I realized I was waiting for the roadster FP to stop, too. :rolleyes:

[AZPete]

Bill, it looks like these guys have all agreed that you should have an OBDII code reader on hand when you start your engine. Do you want to borrow mine? I'm not using it now because my FFR crib is still empty.
Pete

[OCCPete]

Curious: if your fuel pump continues to run wouldn't that eventually burn it up? I thought there was some protection device that caused the pump to stop when your line got up to pressure like some sort of feed back device.

Thank you for your posts, WEK.

When you turn the key to the "on" position, the computer activates the fuel pump for a few seconds to get the fuel pressure up. If my car has been sitting a while, I usually cycle the pump a few times by turning the key on, waiting for the pump to stop running, then turn the key off and back on again. The fuel pump won't keep ruinning unless the computer thinks the engine is running. If the computer thinks the engine is running when it's not, you either have a bad ECU, a wiring problem, or numerous bad sensors.

On the '88-'93 Mustang EFI system, there is no fuel pressure sensor, so the computer doesn't regulate fuel pressure. This is regulated mechanically by the fuel pressure regulator. When the engine is running, the fuel pump runs continuously and the FPR allows excess fuel to return to the fuel tank.

Pete

[skullandbones]

Good afternoon,
I am more confident now with the inof you guys gave. Thanks, CapeCoral and OCC. Also, thanks AZP, but I think I will need this tool in my box for insurance. I have actually been eyeing one but didn't think about it for this project. I got an inspection camera instead but that's another story.

This did bring up an issue. I don't have a place to hook up my fuel pressure regulator on my stock fuel rails. I guess that loop was lost when I abandoned the donor harness. But I don't think that will keep me from starting the engine and breaking it in. I guess I will figure that one out later.

Thanks again guys as always you come thru! WEK.

[Gale K]

I think (going from sketchy memory) that it's still ODB I in '92. ODB II came about 94, 95ish range.

I do know that you can grab the codes from ODB I using a volt meter or the check engine light counting pulses, but the reader makes it easier.

[CapeCoralCobra]

I think (going from sketchy memory) that it's still ODB I in '92. ODB II came about 94, 95ish range.

I do know that you can grab the codes from ODB I using a volt meter or the check engine light counting pulses, but the reader makes it easier.

Yes, OBD I. Book says its good up to '95 Mustangs.