New PCM - now bad gas mileage

[Black91SHO]

I put a new PCM/ECU in my '91 and my gas mileagewent to 15-20 mpg. It was around 20-25 befoee. I put an L0S in it, which is stock for the car. Then I tried another L0S. Same thing...really bad gas mileage. Any idea what's causing that? I didn't do anything else to cause the mileage drop.

 

[SHOdded]

Pics of PCMs old & new?

 

[rubydist]

how long has the "new" pcm been in the car?
any codes at all?

 

[Black91SHO]

I tried replacing my stock PCM that went bad with a used PCM first, and the gas mileage was awful for a couple of weeks (never got better). I assumed it had bad programming since it was on a supercharged car before. So I changed it out with a rebuilt L0S from Ford. Same problem...bad gas mileage.

Pics won't help since the PCM looks practically new.

No codes are being thrown and the car runs fine.

The new PCM's been on for 5 days.

I'm starting to think there's something else causing the bad gas mileage, but the PCM's the only thing that changed. Does it take a while for the computer to learn and produce decent gas mileage?

 

[rubydist]

it does take a while for the pcm to learn things like fuel trim, iac settings, etc. If your fuel injectors are way off "normal" it could be running really lean and that usually results in poor fuel economy, or could be running really rich and that results in poor fuel economy, for example.

how many miles have you put on in the 5 days? it will take at least 100-250 miles to get the pcm dialed in.

 

[Black91SHO]

I've driven it a few hundred miles in the past few days. I drove with the first replacement computer for probably twelve hundred miles or more over a few weeks. I thought its gas mileage was getting better, but I was mistaken.

I've been wondering if the fuel injectors (or something else besides the computer) somehow were affected by the electrical fire I had. Someone who had the car before me rewired a Gen 2 starter into the car, bypassing the Gen 1 starter solenoid. When it burned, it seems it burned the original computer up, just a little. That's why I replaced it. But as soon as I did, gas mileage became awful. I replaced the fuel pump before that, but that didn't reduce gas mileage. It was only after the computer was replaced that gas mileage dropped drastically.

I'm so tempted to sell this car and go back to using my old one. It had it's issues, but nothing like this one. I just hate that this one has so much more stuff on it and I've put so much money in it to fix all of its issues. With my luck, I'll get everything fixed and sell it and the next guy will have no problems with it. lol I hate to give up the Quaife, Cobra brakes, and 3rd gen subframe this one has, though. Those are pricey mods. And I don't want to swap them to the other car if I don't have to...

 

[AREA 91]

Are your secondarys working properly?
Did you remove anything under the hood when replacing the cpu?

 

[sperold]

Just to clarify, you had an under hood fire, you changed the burnt wiring, and at the same time, replaced the EEC, also know as the PCM and ECU. When the repair was done, the bad mileage symptom appeared.
If you put the slightly burned EEC back in the car, would it run?
The EEC runs everything, but it needs wires to get the message out, you may need to look into a new engine wiring harness, but it would be helpful to get the answer to the first sentence.
There are 60 pin positions on the EEC and only 7 are unused. You may be in the right area, but maybe the wiring is the issue

 

[Black91SHO]

Are your secondarys working properly?
Did you remove anything under the hood when replacing the cpu?

I checked the secondaries right after this problem started. And just in case something changed since then, I check them again just now. Still working fine.

I wasn't the one who replaced the PCM the first time. But no, nothing was removed from under the hood when that happened.

 

[Black91SHO]

Just to clarify, you had an under hood fire, you changed the burnt wiring, and at the same time, replaced the EEC, also know as the PCM and ECU. When the repair was done, the bad mileage symptom appeared.
If you put the slightly burned EEC back in the car, would it run?
The EEC runs everything, but it needs wires to get the message out, you may need to look into a new engine wiring harness, but it would be helpful to get the answer to the first sentence.
There are 60 pin positions on the EEC and only 7 are unused. You may be in the right area, but maybe the wiring is the issue

No, there wasn't actual fire under the hood. Technically, the wires just burned up from arching, I'm guessing. I think whoever did the wiring job originally must have not covered all the wires properly, and two of them crossed or arched. The wire only burned when the car was running, not when it was turned off.

I didn't replace the PCM/EEC for a while after this happened. I replaced the burned area of wires, the starter, and that's it. That worked for a while, then I started having a problem. The car would just die randomly, or it would drop RPMs for a split second and pick back up again, the same symptoms as a bad crank sensor (a problem I'd had with one of my other SHOs not long before that. I replaced the crank sensor and found that it didn't fix anything. The problem persisted. So I eventually replaced the PCM (which smelled burned) and it fixed the stumbling problem, but the gas mileage problem started occurring at that time. Around the same time, I had to tighten a hose at the fuel pressure regulator, because it was leaking. It's no longer leaking, and the pressure regulator works fine when I test it, so I don't suspect that's it.

If I put the slightly burned PCM/EEC back in the car, yes, it would run, but it would stumble again, just like before.

I don't think the wiring is the problem, only because this problem didn't happen until I replaced the EEC/PCM. Unfortunately, the part of the wiring harness that burned isn't the part that can be easily replaced--it's part of the car's big harness, not the detachable replaceable part of the harness. I had the same thought a while back just to replace the harness, but it wasn't the replaceable part. No way am I replacing the main harness.

Confused as much as I am yet? lol

 

[Black91SHO]

By the way, I figured it was running rich since it's using so much gas and the engine seems to be staying cooler than I'd normally expect. The extra fuel is unburnt and ends up having a cooling effect. Lean would mean it would run hotter, and I haven't seen that so far. However, I haven't heard any detonations from the exhaust, so far, which I'd expect to hear from time to time if it were running rich. But maybe it's only running rich when I'm on the gas, not when it's idling. I don't know, really.

 

[luigisho]

Well is it throwing any codes? I'd say it's time to probe all the damaged wires, but an easy code to failed part would be nice. Wiring problems are starting to increase a lot lately. Must be the age and all the heat cycles. My least favorite is to track electrical. Got rid of the last car for that reason. Not enough time to spend huge man hours chasing something on a $2k max value car

 

[Black91SHO]

Nah, it's not throwing any codes... I wish it would. That'd make it so much easier.

If I can't figure it out soon, I'll have to check the wires. At least I know which wires it affected and can check those. But I still get the impression that won't be the problem. Who knows. I wish I could just put the burned PCM back on it and see if the gas mileage goes up again. But that PCM's in a landfill right now, and if I put it back on, the car would be dying sometimes and, on rare occasion, not starting again for a little while.

 

[sperold]

Your spout connector (shorting plug) is awfully close to the PCM location.
The symptoms match (if it is a bit doggy, as the timing is locked at 10 degrees), so you could hook up a timing light and observe.
The shorting plug end may have come off when the harness was moved.

There are a variety of EEC units that are reported to be compatible for the 90 (B9B1, L0S and X2J). Did you replace the smelly one with the same unit type.
You are probably not in the zone yet, but keep an eye on your cats. If they get too much gas, the unburnt gas can ignite in the converter and they will glow. But like I said, your mileage is still good enough to keep you out of that zone.

 

[rubydist]

you say that the part of the harness that burned is not the replaceable part, but all of it is replaceable - where are the burned wires?

 

[Black91SHO]

you say that the part of the harness that burned is not the replaceable part, but all of it is replaceable - where are the burned wires?

LOL No, what I meant is "easily replaceable." The part that burned is held snug against the sidewall over the transmission.

 

[Black91SHO]

Your spout connector (shorting plug) is awfully close to the PCM location.
The symptoms match (if it is a bit doggy, as the timing is locked at 10 degrees), so you could hook up a timing light and observe.
The shorting plug end may have come off when the harness was moved.

There are a variety of EEC units that are reported to be compatible for the 90 (B9B1, L0S and X2J). Did you replace the smelly one with the same unit type.

I don't understand how the spout connector can cause this problem. Could you explain?

The performance is decent, but maybe you're right...maybe the timing's off now.

I put an L0S in it, the computer that came in the car stock. It's a '91. I wanted an X2J but couldn't find one for sale.

 

[sperold]

With no timing advance, just initial timing at 10 degrees, you are not going to get good mileage.
It is a long shot, but can be verified easily, so why not try?

 

[Black91SHO]

With no timing advance, just initial timing at 10 degrees, you are not going to get good mileage.
It is a long shot, but can be verified easily, so why not try?

Heck, I'll try it. Sounds like a good test.

 

[bodyman]

If its running cooler it could be that the thermostat stuck open, that may hurt your mileage. Or if the temp sensor is bad and the pcm thinks its cold that may do it also.