Hello everyone I have an 86 with cast heads that has been in an accident and rebuilt about 20 years ago.
I recently started having a problem where I sometimes have a really rough idle, lots of engine vibration and absolutely no power. It even dies in drive sometimes because the idle is too rough. It seems to come on at random, either on the highway or city cruising.
Because of this I replaced the rotor, rotor cap, spark plugs and wires and adjusted the timing by unplugging Est and adjusting with a timing light. I also replaced the Egr and Iac valve, the Maf, cleaned the throttle body, adjusted Tps and idle speed following the fsm procedure. I also replaced the fuel filter, fuel pump and fuel pressure regulator. This car was neglected in its past and it made de car run so much better, but did not fix the issue.

Today while driving the rough idle happenend again, and I tried grounding the alcl connector while driving, entering field service mode and this seems to have fixed the issue. I know it's not good for the engine to run for long periods of time in field service mode but at least I managed to return home.
So, according to this, it seems to me that the ecm might be at fault here. Is there a way that I could test the ecm to see if it is faulty? Has anyone ever tried Cardone refurbished ecms from Rockauto? They seem to be the only affordable solution here in Canada.
Thanks
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Check the fsm to find all of the engine and ECU chassis ground wires, make sure nothing is brittle or frayed you can find used ecu on third gen.org as well as the Facebook tpi tuning groups, Facebook l98 tpi parts group page. Cardone used to be quality stuff, anymore I'm not sure if they are just Chinese crap. You might also check the connection at the ECU and verify the eprom chip isn't loose. Not saying the ECU don't go bad occasionally, but it's not very common for them to fail. Have you had a tech1 scanner plugged in and tried to watch the data as someone else drives to see what it's doing when it acts up?

 

No sadly I don't have access to a scan tool, and of all the mechanics I called no one seemed to know what I was talking about. Ill try checking the connection at the ecm and the grounds. The main one near the oil filter has given me trouble before.
Computer guys sometimes put their computer parts in the oven to melt the solder and repair the connections, are you aware if this could work with the ecm?
Otherwise i'll just bite the bullet and find another ecm, theyre not that expensive

 

Looks like you have done everything except what my initial gut was telling me---- injectors. If you have original injectors, this is most likely your problem. Gasoline has changed a lot in the past 30 years and your injectors NEED the older stuff (there is lubricant to keep them from overheating) to run properly.

These are not too bad to replace on a L98 and I'd look at FIC for a set.

 

I checked the injectors resistance a while ago, they were all within 1 or 2 ohms of each other, cold and hot. I also replaced the o-rings and everything, they don't seem to be leaking. I heard the multecs had problems but I though the 86 were fine with newer gasoline.
Fuel pressure is good, 38psi at idle and jumps up when giving gas
I did notice something interesting tho, when the rough idle situation happens, my fuel economy dips down to 6-7mpg on the dash cluster. It also smells like raw gasoline. This tells me this is not a fuel issue. What do you think?

 

Sorry, this confuses me.... smells of raw gas but not a fuel issue? Please expand on that one. Now, if you have a cylinder down, you will get falling idle, gas smell and of course bad economy. The L98 is bank fired so you will get fuel all the time at the same time. The next thing I'd do is check for spark. I know you have replaced pieces, but you may want to go back to baseline and start over again, just to be sure. BTW.. Multecs ARE a problem.

I know your original request was on the ECM. If everything checks out on ignition, plugs, injectors (they will fail when hot) then consider the ECM. You can take it out and have it tested. I'd hate to see you continue to throw parts at the car.

 

I meant that the car is clearly getting more than enough fuel to run. Could an injector issue be intermittent and not heat related? The issue happens both when the engine is cold and hot.
And it doesnt explain why grounding the alcl fixes the idle and makes the car run smooth again.
And I also meant my car does not have multecs, it has the other kind (I forgot the name) which I heard were reliable

I did check spark again, to make sure, everything seems good.
Could it be a timing advance issue? Like the ecm retards or advances the timing so much that the engine barely runs? According to the fsm grounding the alcl with the engine running makes it so the distributor has a fixed spark advance

 

Very well could be timing issue. It costs nothing to fool with different settings. What do you have it on now?

 

Welp after playing around with the car today the rough idle happened again, and this time grounding the alcl did nothing.
Spark is still good, timing is about 6*btdc (The balancer slipped on itself, I have a new one coming tomorrow) unplugging esc or est did nothing to help.
I removed the intake tube thingy where the maf is attached and I could feel massive amounts of vacuum from the engine with my hand. It almost pullled my hand inside the plenum lol. This seems to be way too much vacuum, and it does not have this much when the car runs great.
The vacuum whine almost drowns out the sound of the engine because it is so loud. The engine is usually pretty quiet when it is running great. I did not notice any vacuum leaks either
I'm picking up a used Snap-On MT2500 off of FB Marketplace tomorrow with all the stuff needed for our cars. I hope it'll help me understand what's happening

Edit: I also checked the oil filter ground connections and everything checked out. I also removed the ecm from the car to look at the components, everything looks good. It's a Delco Electronics remanufactured 16198259. I reseated the Prom and checked the ecm connections for corrosion. Everything looks good so maybe not ecm related, especially if grounding alcl does not fix the issue after all

 

coolant temp sensor?

where in canada are u

 

According to my new (old) scanner, ecm temp sensor looks to be working and accurate. All the other temp sensors and relays for the fans have been ripped out and replaced by a switch panel on the dash
Didnt have much time to play with it today and it was raining, i'll try running the car longer with the scanner hooked up to see if there's something out of the ordinary
I'm in Montreal, Quebec btw

 

does your scanner tell you what the blm and 02 sensors are doing

 

Also, check fuel pressure regulator--unplug vacuum line and see if fuel is inside the vacuum hose--if so diaphragm is leaking--be very cautious if driving car with vacuum line disconnected--if diaphragm is leaking will spray fuel in engine compartment--been there !!

 

Solved! Thank you to everyone who answered. The issue was a bank of injectors not firing, which meant the car was running on 4 cylinders. One of the tabs that holds the fuse in the fuse compartment was broken off, I simply twisted the fuse and put it back in, and it seems to be fixed!

 

haha good job.

which bank? if it was passenger side bank the scanner blms and o2 wouldnt have picked that up.