Ignition timing jump when checking timing with light.

1992 S10 Blazer, 4.3L, 4x4, TBI (Z)

I was checking my timing with my timing light tonight, and I noticed that it has a little jump in it every couple of seconds. The timing mark would be dead on for a bit, then jump. I couldn't tell if it was jumping advanced or retarded, but it wouldn't not stay on TDC consistently for over 10 seconds. What could be causing this? I assume it has something to do with the ECM controlling the spark, but what would make it go off like that?

Thanks,

Jason

1992 S10 Blazer, 4.3L, 4x4, TBI (Z)

I was checking my timing with my timing light tonight, and I noticed that it has a little jump in it every couple of seconds. The timing mark would be dead on for a bit, then jump. I couldn't tell if it was jumping advanced or retarded, but it wouldn't not stay on TDC consistently for over 10 seconds. What could be causing this? I assume it has something to do with the ECM controlling the spark, but what would make it go off like that?

Thanks,

Jason

Usually caused by not putting it in base time mode.
There is a brown and black wire that has to be unpluged to set base time.
Location under hood or at lower ac-heater housing depending on where the computer is located.
If not done the timing can not be set correct.

I did have it in base time mode. It was doing the jumping in base time mode. Is there something that could be causing this?? Distributor with module (reman) was put in this last July. Last time I tested the coil, it tested good. I don't know what else to check. I've even tried a different ECM. Would it be beneficial to try different injectors?? They are the original ones and have been rebuilt twice. The second time, along with the TBI and regulator, was a couple of weeks ago. I'm trying to figure all this out because I have a reman Jasper engine and I'm not getting any better fuel mileage than before. And the "miss" bothers the hell out of me.

Jason

I did have it in base time mode. It was doing the jumping in base time mode. Is there something that could be causing this?? Distributor with module (reman) was put in this last July. Last time I tested the coil, it tested good. I don't know what else to check. I've even tried a different ECM. Would it be beneficial to try different injectors?? They are the original ones and have been rebuilt twice. The second time, along with the TBI and regulator, was a couple of weeks ago. I'm trying to figure all this out because I have a reman Jasper engine and I'm not getting any better fuel mileage than before. And the "miss" bothers the hell out of me.

Jason

Check it out of base time mode also and see if timing is the same.
Also try another timing light.
Is the computer on fender apron or behind glove box.
Which way or what wire are you using are to place it in base time?
Did it set any codes after base time set?

The timing jumps in both modes. The ECM is up behind the glove box and the wire I disconnected is the brown (tan) and black one just below the dash (gives me a code when unplugging it). I know it's the correct wire. I do not have another timing light to use, unfortunately.

The timing jumps in both modes. The ECM is up behind the glove box and the wire I disconnected is the brown (tan) and black one just below the dash (gives me a code when unplugging it). I know it's the correct wire. I do not have another timing light to use, unfortunately.

Is the timing mark pointer the same in normal mode and base mode?

Also check it on the opsite firing cylinder and see if it jumps there.

In base time, the mark is timed at TDC. When in normal mode, the mark is advanced. What do you mean check the opposite cylinder? Cylinder 4?? If it does, what does that mean??

In base time, the mark is timed at TDC. When in normal mode, the mark is advanced. What do you mean check the opposite cylinder? Cylinder 4?? If it does, what does that mean??

Yes cylnder 4 is opposite firing cylnder.

And can be timed on no 4 the same as no 1 cyl.
Checking it on no 4 and if it does not jump would point to a firing problem on no 1.

Another thing that comes in mind could be a worn dist shaft bushing or gear.
Was the dist checked when motor installed.
Also dist cap and rotor.

Good Luck and let us know how it goes.

I will have to check the no. 4 cyl then. The full dist was replaced after the engine replacement, so the shaft and gear should be fine. If the no. 4 checks out okay, what would that mean for cyl 1? If it stills jumps on 4, what could that mean?

I will have to check the no. 4 cyl then. The full dist was replaced after the engine replacement, so the shaft and gear should be fine. If the no. 4 checks out okay, what would that mean for cyl 1? If it stills jumps on 4, what could that mean?

Hard to say.
Go one step at a time and let us know what it does.
Also I would still try another timing light.

I just checked the number four cyl and it was doing the same thing. You can feel the engine missing also. So what can cause a sloppy firing when everything is just about new???

I just checked the number four cyl and it was doing the same thing. You can feel the engine missing also. So what can cause a sloppy firing when everything is just about new???

CAN YOU FEEL MISS ON THE ROAD RUNNING?

Any check engine lights on or flashing?
Any codes past or present?

Good fuel pressure?

I would start checking fuel for good hot blue spaping/jumping spark from coil and into and out of dist.

ALL NEW PARTS usually winds up being a bad new part.

Can't feel any misses while driving. No codes, past or present. Fuel pressure is normal, around 12 lbs. What's a good way to test the spark? Just ground a wire? Spark tester? I agree about new parts being bad. I just wish there was a bad one to find. I can't find it!!

Can't feel any misses while driving. No codes, past or present. Fuel pressure is normal, around 12 lbs. What's a good way to test the spark? Just ground a wire? Spark tester? I agree about new parts being bad. I just wish there was a bad one to find. I can't find it!!

They make spark testing testers with adjustable spark jump..
Or you can check with a metal insert like a screw driver in the end of plug wire and hold the side of it close to engine metal.
Spark should jump 1/2 in. and have a good blue color and a good loud snap.

Always check when cold and hot and after engne has run a 1/2 hr. or more.

So I finally got to do all the tests today. Vacuum is around 19 at idle and at 2000 RPM. Compression is between 180-185 dry on all cylinders. Spark is good. Bought a tester and the spark has a nice pop, blue in color, and jumps up to and beyond half an inch. I tested the spark with the engine warm also. Same. So where do I go from here? What else can be causing the timing to jump around while checking it with a timing light?

Also, I has said I can't feel anything while driving, like a miss or anything. I do however, notice the voltage dip down randomly. So it is happening while driving.

So I finally got to do all the tests today. Vacuum is around 19 at idle and at 2000 RPM. Compression is between 180-185 dry on all cylinders. Spark is good. Bought a tester and the spark has a nice pop, blue in color, and jumps up to and beyond half an inch. I tested the spark with the engine warm also. Same. So where do I go from here? What else can be causing the timing to jump around while checking it with a timing light?

Also, I has said I can't feel anything while driving, like a miss or anything. I do however, notice the voltage dip down randomly. So it is happening while driving.

Did you try another timing light?

I did not use another timing light. I don't have one available. I can hear and feel the jump even when not using the timing light, so I doubt the light is any problem.

I did not use another timing light. I don't have one available. I can hear and feel the jump even when not using the timing light, so I doubt the light is any problem.

Dist and dist module and computer should control timing.
Check dist gear and shaft and dist module.

The timing jumps in both modes. The ECM is up behind the glove box and the wire I disconnected is the brown (tan) and black one just below the dash (gives me a code when unplugging it). I know it's the correct wire. I do not have another timing light to use, unfortunately.

did you try getting it up on number 1 plug and take cap off and see if rotor is pointing to number 1 in cap?

I guess I'll have to test out the dist. Too bad it's brand new a couple months ago. How much play is allowable with the gear and shaft? How can I test the module?


Scrapper, pretty sure the distributor is in correctly. It would be running much worse if it weren't.

O.K. here is my two cents worth. I saw that the Distributor was replaced, but with what?
An AC-Delco or an Aftermarket Distributor?

If you take the Distributor cap off can you see or feel any play in the shaft when pushing on it with hand pressure (will it wiggle)? You should not have more than a few thousandths movement.

Do you have an AC-Delco Cap and Rotor?

Have you checked your Timing Chain for slop? Excessive timing chain slop will cause your distributor to jump all over the map. It is very easy to check. Here's how:

1. Remove the Distributor Cap.

2. Rotate the Crank Shaft Pulley with a socket wrench in the same direction as it runs normally and watch the needle on the Distibutor Rotor move. I like to line up the rotor with a mark or something.

3. Then, click the Socket wrench the other way and rotate the engine in the opposite direction. Mark the crank shaft pulley before you start and see how much movement there is on the pulley before the Distributor Rotor starts to move.

4. I can't say how much is too much or about right, but if the pulley is moving more than 3/4 to 1 inch or more before the rotor moves, you probably have a sloppy Timing Chain.

So I checked and tested the distributor in every which way I could find. The dist is a reman from NAPA. I also tested the ignition coil. Couldn't find a problem. Everything tested to spec. The cap and rotor were installed with the engine. Couldn't say if they are ACDelco or not. The engine was installed a couple months ago by a shop in SC.

So I checked and tested the distributor in every which way I could find. The dist is a reman from NAPA. I also tested the ignition coil. Couldn't find a problem. Everything tested to spec. The cap and rotor were installed with the engine. Couldn't say if they are ACDelco or not. The engine was installed a couple months ago by a shop in SC.

I would go for another dist from a different brand or batch and use a AC delco cap and rotor.

If no problem found with another dist go for a PCM swap.

Well, maybe I'll get to a point where I can try that out. I'll post again later if I do.

Thanks.