91 Z-28 SES light and fan come on at key "on"

[Whoaru99]

My 91 Z-28 (5.7L TPI) has recently started acting up.

When I turn the key to the "on" position, the SES light comes on and stays on, and the cooling fan runs. Then, when I start the engine, the SES light stays on and the engine runs at high idle around 1000-1100 RPMs.

If I drive the car when it's like this, it feels doggy and if you try to step on the gas much at all, it has a pretty good hesitation.

Usually, if I drive the car for 5 to 10 minutes the SES light goes off and, when it does, the car bucks quite noticeably and then runs normally. Once this happens it seems to run fine until I shut it off and start it again.

I've tried to read the flash codes with the paper clip method but if the SES light and fan come on when I turn the key on, the codes won't flash. However, if the SES light comes on only momentarily and the fan doesn't come on, then I can read the flash codes. In the few times I have been able to retrieve the flash codes, only 12 come out (yes, I've made sure to watch past three 12 codes).

Any ideas what it could be?

[Genopsyde]

wow that is strange. I know the fan is supposed to come on when you turn the key in diagnostic mode to flash the codes. does it do this on a cold start, or a warm start? or both?

[Whoaru99]

Happens both warm and cold start.

[Morley]

You are in limp home mode. There is a serious problem with the ECM or Calpack. Need to get a scanner on it.

[Whoaru99]

The car has had an SLP PROM in it for many years, but I still have the OEM PROM.

Would it be wise, senseless, or risky to put the OEM PROM back in and try that before bringing it in for a scan?

[Genopsyde]

yea, try putting the oem prom back in and see what happens.

Morley, I thought the SES light would flash rapidly when it was in limp mode?

[Morley]

Originally Posted by Genopsyde

Morley, I thought the SES light would flash rapidly when it was in limp mode?

There are 2 different "limp home" modes, one is "true" limp home and the other is "field service" mode. One just turns on the SES and the other will flash it. Field service mode is "invoked" by the operator by putting a jumper across pins A & B and starting the car. The flashing of the SES is actually the O2 sensor cross counts.

[Whoaru99]

Have not put in the OEM PROM/CALPACK yet...

Car was running fine most of today, then coming home from dinner out, the SES light came on and at the same time the speedometer stopped working (went to 0 MPH). The speedometer stop working is the first time that has happened.

It stayed this way all the rest of the way home ~15 miles, and had that same doggy feeling and I believe the transmission was not going into lockup. The exhaust also had a different smell to it, sorta sulfury, but not real bad. I suppose that is from this limp home mode? BTW, the car has only 41k miles on it.

Could not retrieve any codes when I tried, the SES light wouldn't flash with the A & B pins jumpered.

Will run through the Service Manual diagnostic procedure tomorrow. Looks pretty simple to ascertain whether on not it's the ECM. Sounds pretty likely though...

I suppose an ECM costs a small fortune?

[Morley]

Originally Posted by Whoaru99

the speedometer stopped working (went to 0 MPH). The speedometer stop working is the first time that has happened.

It stayed this way all the rest of the way home ~15 miles, and had that same doggy feeling and I believe the transmission was not going into lockup. The exhaust also had a different smell to it, sorta sulfury, but not real bad. I suppose that is from this limp home mode? BTW, the car has only 41k miles on it.

Could not retrieve any codes when I tried, the SES light wouldn't flash with the A & B pins jumpered.

Will run through the Service Manual diagnostic procedure tomorrow. Looks pretty simple to ascertain whether on not it's the ECM. Sounds pretty likely though...

I suppose an ECM costs a small fortune?

The speedometer not reading is not an effect of the ECM going bad. The transmission will not lock up the torque converter if there is no signal from the VSS. And since your speedo wasn't working, you had no signal from the VSS.
The sulphur smell is from the cat converter, probably running a bit rich with the goings on you have.
If the ECM is bad you usually won't get the SES to flash any codes.

You can get an ECM on E bay sometimes. Here's one (only found one) http://cgi.ebay.com/ebaymotors/90-92...spagenameZWDVW
You need a "730" ECM. 730 are the last 3 digits of the GM service number. It is also known as a "speed density" ECM.

[Whoaru99]

Did a bit of research on the 730 ECM. Seems it's pretty common on a fair number of late 80s - early 90s GMs.

A few salvage yards around the area claim to have ECMs that will work. Dunno if they're 730s or an interchange, but they're priced $20-$50. Or, yes, there are several 730s on Ebay for ~$45 inc. shipping. Worst case looks like about $95 (after core) for a reman AC Delco.

I'll be checking with the salvage yards tomorrow.

[Whoaru99]

Well, seems like it's fixed. Of course, time will tell but it seems to be working now and I can read the flash codes (all 12, thankfully).

Replaced original ECM 1227730 with 16198262 which is supposed to be a newer service part, and re-used the SLP calpack.

Whopping total....$25 for ECM, $4 gas, and 3.5 leisurely hours R&R, including following the service manual procedure to confirm ECM, a couple phone calls to salvage yards, time to go get the ECM, and a bit of computer researching.

Thanks guys!!