Springster

This is what I found leading me to think the ECM got glitched but I am happy to be told otherwise. I'm just looking for confirmation.
Frayed fan relay wiring at the plug sometimes shorting black|red to green|white (backfeeding the ECM fan driver IC and making a lot of electrical noise I imagine). Relay was chattering. I fixed that and ECM can still drive fan relay when ALDL A to B is jumpered and canister purge valve but when cranking (probably not right). CCP driver IC is same as fan. But maybe the ECM got messed up from the electrical noise?
This is a 1986 2.8L MPFI 1227302 ECM, 65k miles, no rust, very clean inside and under the hood, little to no corrosion. The symptoms: Was running and driving with a slightly rough idle but then wouldn’t start the next day. It fires from the cold start injector (CSI) and a bit from the injectors (if you keep cranking after the CSI has timed out) but won’t keep running.
ICM tested ok several times at 2 parts stores. New ICM made no difference.
ECM is getting the reference pulse from the ICM and is running the injectors (Autoxray/WinALDL shows 4-9mS).
Cat was removed and made no difference.
Plugs were fluffy carbon fouled but changed which made it fire better. Plug wire, cap, rotor are new. Timing seems correct. It should not have changed overnight sitting in a garage. Fires nicely from CSI.
Unplugging the MAF makes no difference. I tried another MAF from a running car and a new MAF and there was no difference.
Forced limp mode of 3.9k on ALDL A to B made no difference (but maybe I didn't do it correctly). I also grounded the Limp pin on the micro and that made no difference. It is always high.
No codes set. TPS at 0.5v.
Check engine light does the “1 blink” then steady on when the key is turned on so EEPROM must be good. ECM can talk out the ALDL.
Grounds measure OK. 12v junction near battery is good. Wiggling wires made no difference.
Found schematics and measured sensors at the micro pins. Probably was a waste of time since Autoxray and WinALDL showed normal sensor values.

So I am stumped from the problem being a partially running ECM or something else. The ECM responds to a 10K and a shorted ALDL A to B. Anyone have thoughts on what else I can check?

Tuned Performance

iTrader: (92)

The ecm sounds fine, this could be bad injectors, low compression, weak coil or high resistance plug wires.

Springster

Ok, thanks. I was having a hard time deciding if the ecm was bad because uplugging sensors seemed to make no difference.

The plugs were carbon fouled when I got the car. I put new ones in and it fired better but after about 50 cranks trying different things the new plugs are now black fluffy carbon fouled. I was assuming it was not starting because it was too lean. Now I think it is flooding. Because: With the cold start injector unplugged, I crank and it fires up then slows down and dies. Crank again it fires a shorter time. Each crank it runs less. After about 5 attempts it doesn't fire at all. Let it set 5-10 minutes and it will do the same thing. Fuel pressure is 42PSI.

The coil output will jump at least an inch to ground. The primary and secondary resistance checked ok and was compared to a new coil.

The plug wires (new) measured about 7k ohm each. I didn't buy them so I dont know if they are the right ones. The cap, rotor and ignition module are new.

Compression was 140 on each (all plugs in, cranking with throttle at idle).

I'll check each injector spray pattern next if the plug wire resistance sounds right.

Tuned Performance

iTrader: (92)

Sounds like you have checked everything. The wire resistance is normal. It could be a bad ecm but having the ecm being able to blink codes is a good sign that the ecm is communicating. You can get a 302 ecm for pretty cheap on ebay but make sure you swap in your prom.


http://www.ebay.com/itm/CHEVY-ECU-ECM-1227302-AJUD-see-item-description-/180981622563?pt=Motors_Car_Truck_Parts_Accessories&hash=item2a23586723&vxp=mtr
http://www.ebay.com/itm/86-Chevy-Olds-Engine-Computer-EBX-ECM-1227302-/230897618230?pt=Motors_Car_Truck_Parts_Accessories&hash=item35c2920d36&vxp=mtr

I didn't see if you have ohmed the injectors , doesn't sound like you can even warm the engine up but heat does make a difference on your readings. The stock injectors have a tendency to short out with newer fuels.

Springster

Ack! Sorry, I forgot to mention that the ecm was replaced but made no difference.

Each injector is 16 ohms but are stock. They hold pressure for around 20 minutes (42 down to 25PSI).

I'm gonna make sure the IAC isn't stuck closed or passageways blocked. Too little air flow would give a rich condition but I dunno if if would prevent starting.

Tuned Performance

iTrader: (92)

The injectors seem in spec. Unless mechanically stuck open or closed from varnished fuel or corrosion . Or the ignition timing is off. If the iac was shut you would need to start the car at 1/4 or so. Open the rpm would just be high.

this_one08

My in-laws car had similar symptoms, he shorted out the whole car installing a long stud on + batt terminal... Installed ecm from pull apart and still idled rough, his maf sensor was shot so got a sensor from pull a part and it runs like a champ

Springster

Wow, that got fried!

Springster

I just wanted to close this thread by posting the problem I found which you'll find with searching for "old varnished gas issue".

The injectors where dirty and not atomizing the fuel.