weird timing! help!

i have 1994 suburban 5.7 2wd. had issues whereit didnt want to run,then ran at high idle. there was a code stating cant set idle,replaced iac motor twice same problem then replaced ecm with known good one and same problem. one day it stopped and decided to barely run. decided to check timimg and it seem off so i disconnected timing wire and set to 0 deg. plugged connector back in and the mark stayed in the same place at 0 deg. cut engine off and restarted it and the timing mark went away like it was suppposed to. now it is back to 0 deg on its own while connector is connected but while watching the mark while running (up and down idle) the timing mark moves back and forth damn near a half inch and it should be still! can anyone out here can give me an idea of where this problem could be coming from? both ecms do the same thing.

i have 1994 suburban 5.7 2wd. had issues whereit didnt want to run,then ran at high idle. there was a code stating cant set idle,replaced iac motor twice same problem then replaced ecm with known good one and same problem. one day it stopped and decided to barely run. decided to check timimg and it seem off so i disconnected timing wire and set to 0 deg. plugged connector back in and the mark stayed in the same place at 0 deg. cut engine off and restarted it and the timing mark went away like it was suppposed to. now it is back to 0 deg on its own while connector is connected but while watching the mark while running (up and down idle) the timing mark moves back and forth damn near a half inch and it should be still! can anyone out here can give me an idea of where this problem could be coming from? both ecms do the same thing.

back2life,
I don't have an answer for you (yet) but I will let you know that I am fighting the exact same problem. So, if you figure this out, please post back (as well as I will if I ever figure this out).

In my case it is a 90 GMC K1500 Sierra with a 5.7 and 700R4.

The first time I noticed the timing jumping around, I figured my 25 year old Snap on Timing light had finally died. Bought a new light, still had the problem. Replaced the timing chain since I was convinced that was it. The chain was very loose but still had the problem.

Rebuilt the dist (new shaft, module, pickup coil and drive gear) still had the problem.

Just last night, I disconnect ESC, 02 and the MAP sensor and the timing is still moving around. WTF!

I have a MSD Coil in it right now and I have put the stock one back in and the problem occurs with both of them.

I replaced the ECM because I was convinced that was my issue (and the circuit board looked a little burned in one area) and it seemed to help; however, with the recent warmer weather (and rain), the problem is more prominent again.

The wires were replaced about 25K miles ago (again MSD) and they check out with an ohm meter. Plugs, cap and rotor are all new.

It pasted smog (California Level 2) but the hydrocarbons barely passed. I know that the valve seals are leaking (puff of blue smoke at startup) but I don't understand how that could cause the timing to jump around at idle.

Compression is decent (170 +/- 5). Oil pressure is good (25 at hot idle). Fuel pressure is 13 PSI. Vacuum is 20.

I recently found that the MAP sensor vacuum hose was bad and it ran good after replacing it; however, the idle problem got worse after the warmer rainy weather.

Cat and exhaust are original. Exhaust backpressure was less that 1 PSI at idle so I don't think it could be a clogged exhaust.

02 is a brand new AC Delco. I have WinALDL and the BLM says that it is running a little rich (125 and I have seen it as low as 108).

Sorry this is long winded but I have been frustrated with this problem. I may work on it later today and if I find anything I will post.

Hopefully, someone who knows what might be the issue will post in this thread. I know that the ECM controls the ignition timing so it makes sense to me that the problem must be with the ECM, dist (pickup coil and ignition module), coil, worn dist shaft/bushing, cap or rotor.

Good Luck,
2000CAYukon

i forgot to add vehicle had sat for two years with a no start issue from sloppy timing chain(u might remember my previous posts)when i found one to fit right it was a competiton style chain(looks doubled) we could not get it to run right and one minute had spark and next it didnt. i put in new ac delco plugs,distributor,came with new module(rebuilt old one before i done this and had no start),cap and rotor,coil started instantly timed it and it ran like a champ,until the new problems arose. parts did come from autozone tho. i never seen the mark jump on anything i've ever timed before! i also put on a new cat when i got her running afterwards. muffler bout 3-4 years old. do u think it might be arcing across inside the cap,making #1 cylinder fire more than its supposed to? im betting on distributor or that damn ignition module cuz it wont advance when putting connector back together until i cut it off and restart it then it advances or wherever,but i can still see the mark about an inch from 0 deg mark it i used to be somewhere i couldnt. someone also told me the module?distibutor is what controls timing the ecm monitors the info:banghead:

99% of these type of problems came from the pickup coil ring on the dist shaft. It would crack and let the pickup coil move on the shaft causing the timing to jump around. back2life, id replace the distributer with a reman. 2000CAYUKON, what dist do you have in now?

99% of these type of problems came from the pickup coil ring on the dist shaft. It would crack and let the pickup coil move on the shaft causing the timing to jump around. back2life, id replace the distributer with a reman. 2000CAYUKON, what dist do you have in now?

J-Mech,
I have the original dist with a new shaft (magnet was loose), new pickup coil and new drive gear. I tried a second ignition module but that did not change anything. There does not seem to be any play in the upper shaft bushing.

The thing here is that the timing does not move when the dist is in bypass mode. If the pickup was bad, I would think that the timing would still jump around in bypass mode.

I wish I had a scope so that I could see the square wave coming from the ECM (which is a new reman unit). I am also tempted to try a new ignition module since I suspect that could be part of my problem. The vents were clogged and I may have cooked both ignition modules over time.

//2000CAYukon

thanks j-mech! my distributor is a remanufactured unit,came with a new module also. few months old is there a way of testing your theory without having to spend excess money?:banghead:

back2life,
Any updates on your problem? I know that 99% of the time it is the dist; however, I must be one of the lucky ones. I decided to replace the dist (due to the fact that with the EST disconnected, the timing seemed to jump every 20 or 30 seconds). I installed a brand new dist from Jegs.

It does seems to be running a little better; however, the idle problem is still there. This morning, I hooked up WinALDL and the BLM was 108. This indicates that the ECM thinks it is running rich so it is removing fuel.

The cat is original (17 years old and 125K miles). Can this be the cause of my issue? The truck passed smog a few months back but barely. It had the highest HC count that could pass.

The injectors have a nice cone pattern; although, I do notice droplets of fuel on the butterflys when it is idling.

Fuel pressure is 13 PSI and the regulator was replaced recently.

I also have bad valve seals (blue smoke at startup) and I have not had time to replace them but will soon. Can this cause the ECM to think it is running rich?

The idle issue is intermittent but it seems to get worse after the truck has been driven for around 30 minutes (bad cat?).

Any ideas or suggestions?

//2000CAYukon

no i havent checked anything. just been driving, i will get to it soon got to much other stuff to do. by the way what is winaldl?

by the way what is winaldl?

See http://winaldl.joby.se/

It is basically a low end scan tool that runs on a PC/Laptop. I had to buy a cable (www.akmcables.com) but I have used it on a few different vehicles.

If you find or fix anything please post back and I will do the same.

//2000CAYukon

i usually only get on here on weekends because of time. either one of you had any luck yet?

i usually only get on here on weekends because of time. either one of you had any luck yet?

I spent some time on mine this past weekend. I now think my whole problem is engine mechanical not ignition or ECM like I was thinking.

I have the dreaded blue smoke at start up and I believe that I probably have a bad valve guide or 2. If I reset the ECM (unplug it) and start it up, BLM is right at 128. If I drive it for a while, the BLM will head south. This morning it was at 125 after getting to work; however, I have seen it at 108 last week and the idle is real rough when this happens.

My theory is that due to the valve seals/valve guide, the ECM thinks it is running rich so it removes fuel and the truck is actually running lean and I get a misfire (at least that is what it feels like) at idle. The truck seems to run fine at full throttle since the ECM is in power enrichment mode. I do not get any smoke while running so I think the rings are OK.

I bought new valve springs and valve seals a while ago and just never installed them. Now I have to decide if I want to pull the heads (valve job) or just replace the seals and springs. I know the stock heads are not that good (flow wise).

I have owned this truck since new and it has been well maintained. I think the bottem end is in good shape (good oil pressure and compression) so a top end refresh should do it good. Right? My concern is doing the top end only to find out that it really needed the bottem end as well.

My goals are just to have a good running truck. I am not looking to add a lot of HP to this engine; however, it is needs a new engine than I will probably replace it. If I had more time (married with kids), then I would not hesitate to yank the engine and rebuild it. The problem is that I don't have the time so it will probably be a stock replacement engine.

Top end or rebuild? That is the question!

//2000CAYukon

I'm hesitant to think that with a blm of anything over 100 would create a lean misfire. I also cannot imagine that you are getting enough oil into combustion that the o2 sensor is picking it up. Have you pulled the throttle body and cleaned it? How about a new PCV valve?

I'm hesitant to think that with a blm of anything over 100 would create a lean misfire. I also cannot imagine that you are getting enough oil into combustion that the o2 sensor is picking it up. Have you pulled the throttle body and cleaned it? How about a new PCV valve?

J-Mech, my concern is not oil getting into the combustion but unburned fuel getting past the exhaust valve and fooling the ECM that the engine is running rich.

Throttle body is rebuilt and clean. I bought some reconditioned injectors so that could still be an issue.

I don't remember when I changed the PCV last so I will look at that. The gromet needs to be replaced (has gotten hard). Hose is in good shape (tested with a vacuum pump).

The weather has gotten a little warmer the last week and the idle quality has gotten better. Idle is still a little rough but much better with the warmer weather.

I don't have a leak down tester so I ordered one and plan to do a leak down test this weekend to see what shape the engine is in.

I will post back the leak down test results on Monday.

//2000CAYukon

How about a new PCV valve?

J-Mech, thanks for pointing me in the right direction. The PCV valve that was in the truck was a Fram piece. I picked up a new one (AC Delco this time) and also replaced both gromets (one on each valve cover). The PCV gromet was hard as a rock and probably was not sealing.

Right now, the idle quality is a whole lot better. I checked BLM after the drive into work this morning and it was a 122 (much better than 108!).

I plan to drive it for a week and see how things go. I did not get a chance for the leak down test but at this point, I don't think I need a valve job. I would rather use that money towards a new cat and cat back exhaust (I am still running stock cat and exhaust).

back2life, you should make sure that your PCV valve is sealing well on the valve cover gromet.

Thanks again J-Mech and I will post back after a week of driving.

//2000CAYukon