1991 6speed doesn't rev smoothly, and backfires and bucks like crazy when I try to drive it. I plugged in a diagnostic cable and it indicated that the spark is advanced to 45+ degrees before when I rev it to 1500 RPM. Base timing is set at 6 before, and when I unplug the EST wire the car revs fine. I haven't driven it with the wire disconnected but any roughness while revving is gone, its a noticeable difference. Any ideas of what might cause this? Bad ICM maybe?

 

I drove the car just now with EST unplugged and it drives perfectly, actually even better than it used to.
I just got the PROM replaced as my old one failed, and my guess is a previous owner deleted some emissions equipment that I don't know about.

I know the EGR is deleted, and I have that programmed into the new PROM. Is there any other emissions stuff that effects timing?

 

The things that affect timing are TPS, MAP, CTS, EGR temp sensor, primarily, if not entirely. Maybe IAT too.

EGR coincides with more timing, but that is code, not a sensor. When the ECM commands EGR, it adds timing. But 45 degrees? IDK...that is a lot. Is the PROM custom tuned or OEM? Have you verified ECM claimed timing with a timing light?

 

All the sensor readouts look good from my tuner cable. Don't see anything out of the ordinary. I haven't checked with a timing light, but I don't think I would be able to see anything above 10 degrees before, because I can't see any marks on the balancer besides TDC. They might be there but if they are they're real hard to see.

I got the PROM from gmeprom.com, they loaded it up with the stock program, then disabled the EGR and set the fans on to 210 at my request. So it was custom tuned but it should have the OEM program on it? I'm not exactly sure.

He says it's MEMCAL AXCN 9806.

 

Sorry no help here...here's the lower spark map table for AXCN. If you really think that's the issue, you could pull back a few degrees on the initial 6* to see if it improves. I can't remember what has been done from the beginning of your issues but if everything in the ignition system is allegedly new i.e. cap, rotor, wires, plugs, ICM, coil...I seem to recall reading in a magazine's "tech section/what's your problem" and the response to customer's missing ignition issues from a former GM tech "hail Mary pass" was to swap in a new distributor. I'm not one to blindly throw/swap parts before troubleshooting but allegedly, the one last component would be the pickup coil.

 

I tried retarding the base timing but it only decreased throttle response and didn't really do much help. The distributor is only a few months old but I may have damaged it last time I had it out.
I had the distributor out a while ago, and accidentally plugged in the harness connector backwards. The plastic retainer clip thing had broke off because of old plastic, and the connector is down behind the intake manifold where I can't see it and can barely even reach it, so I had to go by feel to get it plugged in. I found out about it because it wasn't sending reference pulses, and plugging it in correctly got it to where it is now (running, but too much advance). It was a dumb mistake but seemed like everything went back together ok. Could this have caused the pickup coil to be fried or something? And is there any way to test the coil?

I do have an old distributor, I think it still has the pickup coil in it, which should still work fine. maybe I can try to swap that one in and see if that fixes it.

 

^That is showing 46* in cells, so maybe the OP's problem isn't the tune.

OP, verify your timing mark on the damper. Perhaps it's slipped and you've actually set the base timing quite a bit further advanced than the 6* that you think you set it at?

 

I haven't verified it, but I did retard the base timing to see if that helped, but it only made the engine run worse and made pedal response pretty terrible, and caused some backfiring. It definitely runs much better at 6btdc according to the damper mark, leading me to believe it hasn't slipped.

My thoughts are that maybe the spark advance is correct for the oem engine, but there's something different about my engine that's causing the mixture to burn quicker, so it needs less advance? I'm not sure, I know the previous owner did some modifications to it but I have no idea what all was done. Nothing major, just stuff like relocating the IAT sensor and removing the EGR. The old PROM was clearly modified somehow, but the data on it is gone, there's no way to know what was changed. Is there anything that might cause that?

 

I ohm tested my pickup coil and it passed, I don't really think anything is broken. It just seems like the engine and computer aren't agreeing.

 

Alright I attempted to verify TDC.
The damper bolt was SUPER loose when I tried to turn the engine over. I had to turn it quite a few times before it got tight enough to turn the engine (I took all the spark plugs out). Would that cause the damper to slip and lose time?
I tried to verify TDC by feeling for the piston coming up and down, and I think I felt it coming up after the mark passed TDC, but I rotated it around again and it felt like it was coming up before, so I'm not really sure. Do you know a good way to verify TDC? I just put a small screwdriver in the sparkplug hole to feel the piston coming up and down.

What's confusing me though is when I retarded the timing on the distributor the car ran worse, so if the damper had slipped and it was too advanced, it should have improved the condition, right? I don't remember if I plugged EST back in or just tested it with it unplugged, but I think even unplugged it should run pretty well. I'm not sure.

 

You should start fresh. Make sure the dampener isn’t damaged. Get it tight. Verify cyl 1 TDC compression stroke. Set dizzy. Set base timing.

cyl 1 TDC - remember you want compression stroke. Both valves closed. Remove the valve cover if you need to. Pull one or all of the plugs to make it easier. Get a helper to turn the motor while you hold your finger over the plug hole. Use a screwdriver in the plug hole to see how the piston moves. Buy a cheap bore scope on Amazon and use that.

I’m pretty sure the dampener is keyed, so no issue there. They get old and the rubber moves on the hub. This will throw your timing mark off.

 

^This. You just said it at the end of your last post, OP....."I'm not sure". So let's start over and ensure that every thing is correct.

Yep. The screwdriver in the hole is a quick way to check that it's close to TDC. If you want to be more precise, then you need a piston stop. You can make an effective one out of a variety of things.....I'm a fan of the metal coat hangar b/c it's cheap and plentiful. Straighten it (or any stiff wire metal thing) out, then bend a "J" in the end, making the hook of the "J" maybe 3/4" to an inch long? Squish the "J" so that it's narrow enough to fit through the plug hole. Insert the "J" though the spark plug hole, hook the end of the "J" inside the combustion chamber and pull up/out on it to hold it in place. Have a partner hold that or if you're good, turn the engine slowly until the piston is stopped by the coat hangar "J". Put a mark on the outer damper ring at your "0*" pointer. Turn the engine backward just uner 360* (while still holding your tool in place) until the piston comes to a stop against the "J" again. Mark the damper again, at the 0* pointer. Now, remove and discard your hangar/"J" tool, measure the distance between the two marks you made on the damper outer ring, split the distance in 1/2, measure from one of your marks toward the other and mark it at the 1/2 distance you measured and calc'd. This mark should be dead center between your other two marks, and is your actual, TDC.

With that either establish (or confirmed if your final mark falls right on the timing groove in the damper), you can then move forward with setting base timing with the EST disconnected, THEN reconnect the EST and try driving the car.

 

Alright I verified TDC. It's right. The base timing is set at 6 degrees before. I did try to back off the timing, which did nothing, but I noticed something new.

It's cold today, and the problem seemed to go away when I started the car up. It didn't want to rev at all at first, which it has done as long as I've owned it when its cold, but after letting it warm up a little (still not showing on the gauge, maybe a minute or so), it revved fine and the problem seemed to go away. However, after a few more minutes of running the problem appeared again. I also noticed that unplugging the advance did NOT fix the problem with the car cold. It still didn't want to rev, although it did rev better, it wasn't perfect like it was when it was warm. Also the car won't idle with the advance unplugged. It jumps up and down for a little and eventually stalls.
So it kind of seems like temperature affected it. That makes me think of MAP or a vacuum leak.
I'm not sure where a vacuum leak would come from though, as I just replaced all the intake gaskets. A vacuum leak is one of the first things I thought of, and I already went around with carb cleaner and didn't find anything.

 

I plugged in my tuning cable again and everything seems to be working correctly

I did notice that the target AFR was 14.8 at idle, and remained there consistently from cold all the way to OT. When I applied part throttle it leaned out even more to mid-15's (accompanied by the revving issues as usual). This seems really lean to me. I did some research but all I could really find on target AFR's was for WOT. Still, especially for a cold engine, 14.8 seems really lean. Any ideas?

 

Have you checked fuel pressure?

 

Yes fuel pressure is good and holds
The regulator, pump, filter, and injectors have all been replaced in the last 6 months or so

 

I should mention I don't have a working O2 sensor at the moment so its permanently in open loop
tomorrow I'm going to go get a new one and hopefully I can get it to go into closed loop, and maybe that will correct the AFR?
If that is the issue I'm hoping it'll run in closed loop.
Maybe the open loop tables are wrong? I don't know much about how these computers work.

 

I'm not sure if anyone is still following this thread but I went out and ran the car for a bit today and noticed that the problem only exists after the car has warmed up for a few minutes. It runs perfectly fine with the spark advance and everything up until the temp gauge starts moving past the low mark (100). Then it starts stuttering like it has been, and gets worse as the temperature increases.
Could this be a bad ICM maybe? If the ICM was bad would it show up on the spark advance readings I'm getting on the scanner? Because the scanner indicates the correct advance, but I'm not sure if that is actually the advance that is happening or what. I know AutoZone can test ICM's but that test wouldn't account for the heat I don't think.
Also the car idles perfectly fine even when it's hot but I did notice that when it starts to get hot the engine is rougher and doesn't sound as smooth either. It still holds 700RPM just fine but physically I can feel the engine running much rougher as it gets hotter.

The engine starts right up when it's hot. It sometimes takes a couple tries to start when it's cold (It'll get up to 500RPM or so then die, then on the next try it will catch and slowly climb up to 1200 or so, and then it'll hold there for a few seconds before dropping to idle). When it's hot it fires off pretty much the second I turn the key, it's instant.

I'm thinking ICM but I'd like to get some feedback before I go buying stuff.
Thanks

(I got a new O2 sensor by the way. I don't have my laptop right now so I can't see if it's working correctly with the ECM, but it did not change the problem at all.)

 

I think you're lacking feed back b/c you're kind of all over the place with what you actually KNOW. It's hard to offer advice when the story/info keeps changing on what we've been told. For example, we gave advice and you chased a bunch of timing issues.....It seems that in the end, there never was a timing issue at all.

If it runs good until 100*F or so, then like crap, it could be an O2/going into closed loop. When you get above 100* and it starts running poorly, what are your O2 cross counts?

Think of the ICM, functionally as points; it's an on/off relay that grounds and then opens the coil primary circuit. It could be giving you issues? But if it idles fine, hot, it's probably not.

To ME, it sounds like there are a bunch of issues that need to be addressed. IDK what all they are, b/c I'm not there so I can't see/feel/hear the engine to get my own idea of where I'd go first, with it.

 

yeah that makes sense and I apologize, I’m not really sure what’s going on with the car and timing was my best guess from what I knew at the time. I only have an hour or so to work with it every day so I don’t have all the information at once. When I give new information in the middle it’s because I just discovered it myself.

until yesterday it was permanently stuck in open loop. I just bought an O2 yesterday which didn’t change any of the symptoms at all. I haven’t had the chance to plug it into a computer though so I’m not actually sure if the new one is working, but there are no codes.

It does hold idle just fine when it’s hot but the quality is noticeably worse. It feels and sounds much rougher as the temperature increases. When I unplug the EST it won’t hold idle at all. It used to when the problem began but not anymore.

I’m sorry it’s all over the place, I’m a little all over the place I’ll try and be clearer.