Welcome to the Corvette Forums at the Corvette Action Center!

Strange timing issue

KANE

Moderator
Joined
Mar 2, 2002
Messages
3,244
Location
KY
Corvette
Dark Blue 1982 Trans Am(s): Polo Green 1995 MN6
You need a scan tester do you can look at the spark timing in the data list and see what the ECM is commanding as far as spark.

To Hib's point... some type of real time scan would be great.

Datalogging would let you see what is going on real time and then to review it without the car running.

Sunpro has something that interfaces with the car and has playback. Looks similar to an old school Tech 1. It's pretty good- I use those for my OBD II cars, but something laptop based would be better and more mobile.

More food for thought...
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
I will have to take it back to my mech to use his data logger.

I threw in a new primary coil and it's the same issue, so I'm looking at the pick-up coil and the ignition coil now. Everything in my brain tells me this is a spark/timing issue. That just what it sounds like.

I'll order a new FSM. I really hate that my old one is gone. I probably could have used that by now...

Just one question. I've rebuilt Distros before, but I can replace the pick-up coil without pulling the distro, right? I really don't want to deal with that damn oil pump notch again. <_<
 

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
I will have to take it back to my mech to use his data logger.

I threw in a new primary coil and it's the same issue, so I'm looking at the pick-up coil and the ignition coil now. Everything in my brain tells me this is a spark/timing issue. That just what it sounds like.

I'll order a new FSM. I really hate that my old one is gone. I probably could have used that by now...

Just one question. I've rebuilt Distros before, but I can replace the pick-up coil without pulling the distro, right? I really don't want to deal with that damn oil pump notch again. <_<

cali navy, you lost me. I threw in a new primary coil and it's the same issue, so I'm looking at the pick-up coil and the ignition coil now.
do you have the stock HEI? one ignition coil, one pickup coil? I admit that I do not know what a "Distro" is. but I am quite familiar with the HEI, installing, rebuilding, whatever. the HEI has the distributor shaft inside the pickup coil, so it -the shaft- has to be removed. meaning the distributor has to be removed.
if you have HEI you can check pickup coil with a meter. I can give you the two simple tests.

keep in mind that IF you have a strong spark using a GM type tester, the two coils should not be the problem. if you have a yellow spark, I THINK the pickup may be faulty, will have to verify that. I have a pdf file of the 86 chart A-3, "Cranks, won't run," if that will help you.
you can also find useful into in the Haynes Manual, and I am sure that will cause some minor uproar, but it is useful, better than the FSM, for many things. I have both.
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Lol. Sorry Ned. When I'm frustrated, I shorten everything. Distro = distributor.

I have a stock HEI. Basically, I've done a lot of maintenance to the vehicle. Consider it 50K mile maintenance. I did this just to get rid of as many question marks as I could. Cap, rotor and plugs. I replaced the fuel injectors because, well, for the hell of it.

I have used a GM in-line spark tester and yes, it was yellow, not blue. Therefore, weak spark. In fact, very weak spark. That is why I am once again focusing on the distributor as a weak spark issue. I replaced the coil and there is no difference. Therefore, I have ordered (and overnight shipped) a new pick-up coil from Eckler's. I have also ordered a new Ignition Control Module. Why? Well, for the hell of it. It's cheap.

So, the pick-up coil is supposed to be here on Wednesday before 8pm. Hopefully, it will be here before it gets dark and I will get to install it. I really don't want to pull the distro, but we will see. TDC and all. :hb
 

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
Lol. Sorry Ned. When I'm frustrated, I shorten everything. Distro = distributor.

I have a stock HEI. Basically, I've done a lot of maintenance to the vehicle. Consider it 50K mile maintenance. I did this just to get rid of as many question marks as I could. Cap, rotor and plugs. I replaced the fuel injectors because, well, for the hell of it.

I have used a GM in-line spark tester and yes, it was yellow, not blue. Therefore, weak spark. In fact, very weak spark. That is why I am once again focusing on the distributor as a weak spark issue. I replaced the coil and there is no difference. Therefore, I have ordered (and overnight shipped) a new pick-up coil from Eckler's. I have also ordered a new Ignition Control Module. Why? Well, for the hell of it. It's cheap.

So, the pick-up coil is supposed to be here on Wednesday before 8pm. Hopefully, it will be here before it gets dark and I will get to install it. I really don't want to pull the distro, but we will see. TDC and all. :hb

navy man, the "D" threw me off, plus ref to "primary coil." I thought you might have one of those gimmick boxes that people bolt/connect into the system, like MSD, whatever.
I may be wrong about the spark color IF using the GM tester. FSM does not mention color in that test, but does refer to "weak" spark other times. that test requires 25,000v to fire.
but...the HEI depends on a full 12Vdc at the ign coil/pickup coil. if there is resistance to the 12v supply, the HEI produces a weak spark. bad terminal connection, etc, can do it. the spark starts with the pickup, so if I were desperate, I would replace it -or do it anyway, as I did when rebuilding mine, which had no problems before or since.
some tips if you have never done a rebuild:

put timing mark at 0 on tab, with rotor pointing to front. it will be ABOUT a 11.5-5.5 mark on a clock face, just short of the six o'clock position.
make a mark/scratch, whatever on the dist base to manifold, so you can reset it close to correct advance.
the oil pump shaft will rotate CW -counterclockwise- when you remove dist, which is why you have to reposition it when re-installing. NOTE: the shaft slot centerline will always match the rotor centerline, so that it will need to be at the same direction to drop the dist. in.

observe where the shims are under the dist drive gear before driving the pin out.

the pu coil has a thin C washer under it that has to be removed, using small knife blade, whatever. pack the opening under coil with a hard grease. spin the dist shaft to be sure the teeth do not touch teeth on pu coil pole piece.

most of it is self explanatory. the book says use clear silicon grease, thin, under ICM. you will get that or similar with the ICM.

you can align the pump slot using a long screwdriver so that it is ABOUT pointing between the 5 and 6 on the clock. before dropping the dist into the cam gear, rotate the rotor CW 2/3 of the way toward the #2 firing tab on dist cap, because the teeth are spiral and dist. gear has to rotate to mesh. it will settle into place.
OR -with rotor pointing towards #2 as above, you can rotate the engine -I have used a 15/16" socket to turn the alternator bolt, turning engine backwards, and the dist shaft/rotor will drop in at the #6 position. (You can't turn the bolt in the normal direction because of friction.) if you continue turning to align the balancer mark to 0, it will be at #1 firing position.
For 40 years, I have simply rotated the oil pump shaft.

there is only one combination of cam gear/dist. gear slots that is correct for the alignment you want -meaning with dist cap positioned to the factory spec. so if the rotor does not align, you are off one gear tooth. position the dist housing to where your previous marks align and it will be close enuff to start.

if you need further clarification, send a PM, or comment here.

ned
 

ned pepper

Member
Joined
Sep 17, 2012
Messages
15
Location
Indiana
Corvette
91
location of #1 cylinder in dist cap.JPG
Lol. Sorry Ned. When I'm frustrated, I shorten everything. Distro = distributor.

I have a stock HEI. Basically, I've done a lot of maintenance to the vehicle. Consider it 50K mile maintenance. I did this just to get rid of as many question marks as I could. Cap, rotor and plugs. I replaced the fuel injectors because, well, for the hell of it.

I have used a GM in-line spark tester and yes, it was yellow, not blue. Therefore, weak spark. In fact, very weak spark. That is why I am once again focusing on the distributor as a weak spark issue. I replaced the coil and there is no difference. Therefore, I have ordered (and overnight shipped) a new pick-up coil from Eckler's. I have also ordered a new Ignition Control Module. Why? Well, for the hell of it. It's cheap.

So, the pick-up coil is supposed to be here on Wednesday before 8pm. Hopefully, it will be here before it gets dark and I will get to install it. I really don't want to pull the distro, but we will see. TDC and all. :hb

navy man, the "D" threw me off, plus ref to "primary coil." I thought you might have one of those gimmick boxes that people bolt/connect into the system, like MSD, whatever.
I may be wrong about the spark color IF using the GM tester. FSM does not mention color in that test, but does refer to "weak" spark other times. that test requires 25,000v to fire.
but...the HEI depends on a full 12Vdc at the ign coil/pickup coil. if there is resistance to the 12v supply, the HEI produces a weak spark. bad terminal connection, etc, can do it. the spark starts with the pickup, so if I were desperate, I would replace it -or do it anyway, as I did when rebuilding mine, which had no problems before or since.
some tips if you have never done a rebuild:

put timing mark at 0 on tab, with rotor pointing to front. it will be ABOUT a 11.5-5.5 mark on a clock face, just short of the six o'clock position.
make a mark/scratch, whatever on the dist base to manifold, so you can reset it close to correct advance.
the oil pump shaft will rotate CW -counterclockwise- when you remove dist, which is why you have to reposition it when re-installing. NOTE: the shaft slot centerline will always match the rotor centerline, so that it will need to be at the same direction to drop the dist. in.
observe where the shims are under the dist drive gear.
the pu coil has a thin C washer under it that has to be removed, using small knife blade, whatever.

most of it is self explanatory. you can align the pump slot using a long screwdriver so that it is ABOUT pointing between the 5 and 6 on the clock. before dropping the dist into the cam gear, rotate the rotor CW 2/3 of the way toward the #2 firing tab on dist cap. it will settle into place.
OR -with rotor pointing towards #2 as above, you can rotate the engine -I have used a 15/16" swocket to turn the alternator bolt, turning engine backwards, and the dist shaft/rotor will drop in at the #6 position. (You can't turn the bolt in the normal direction because of friction.) if you continue turning to align the balancer mark to 0, it will be at #1 firing position.
For 40 years, I have simply rotated the oil pump shaft.

there is only one combination of cam gear/dist. gear slots that is correct for the alignment you want -meaning with dist cap positioned to the factory spec. so if the rotor does not align, you are off one gear tooth.

if you need further clarification, send a PM, or comment here.
ned (added pic: the blue mark on dist hsng is where your rotor will be pointing as it is now and should be on reassembly. I would mark this position before removing dist.)
 
Last edited:

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Oh, believe me, the grease pencil is my friend here. I have dealt with timing this thing before and I don't like it. :p
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Well, the pick up coil did help. Same symptoms. However, I have confirmed something. Every time I do a computer reset by disconnecting the battery, the car run great for the first 30 seconds of driving. It seems like when it goes into open loop, it starts running like crap. I have gone thru the process with the engine both cold and hot. Same results. Kinda strange. Points me towards electronics. Timing was easy for once. Started right up the first turn and it was only a few degrees off anyways. I'll have the new Ignition Control Module tomorrow.

Distro breakdown wasn't too bad. The pin was a pain to get out and the retaining clip was slightly irritating.
 

Hib Halverson

Technical Writer for Internet & Print Media
Joined
Jan 10, 2001
Messages
13,453
Location
CenCoast CA
Corvette
71 04 12 19
My wild-assed guess is what's happening is exactly the opposite.

When you start the engine, it's in open loop and then after 30-seconds it goes closed loop and runs poorly because something is amiss with the engine controls. It should be lack of spark control but it could be something else.

In the end, you need a scan tester so you can see what the loop status becomes after that 30 seconds.

You need a scan tester which supports 81 and the factory service manual.

I'm too lazy to go back and read all the posts so far, so let me ask you...have you attempted to get any fault codes set in the ECM by grounding the diagnostic pin of the DLC?
 

cali navy

Member
Joined
Dec 7, 2007
Messages
15
Location
California
Corvette
91 non-ZR-1
Lol. Yes, I get open and closed loop backwards sometimes. :p
I'll do a quick overview for the TLDNR.

Stumbling and backfire under load.
Replaced:
Injectors
Distro cap and rotor
Primary coil
Pick-up coil
Spark plugs

No codes
I have plugged it into a logger and all sensors are reporting correctly.
 

Corvette Forums

Not a member of the Corvette Action Center?  Join now!  It's free!

Help support the Corvette Action Center!

Supporting Vendors

Dealers:

MacMulkin Chevrolet - The Second Largest Corvette Dealer in the Country!

Parts/Accessories:

Vetteskins

Advertise with the Corvette Action Center!

Double Your Chances!

Partners

Top Bottom