98 4.3 midrange misfire

I have a 1998 4.3 engine that is misfiring from about 1500 rpms to about 3000 rpms. It is more severe when the engine is cold and it is quite a bit less noticeable when you drive it for 20 plus miles. It is so bad that when the engine is cold the vehicle's front end bounces up and down from the surging. It idles great, and when you floor it.....even when cold, it will not misfire. I have a solid lit SES light that has been decoded to a "multiple cylinder misfire". The things I have done to it so far are: new plugs, new plug wires, new cap, new rotor. The weird thing is after I changed the plug wires it quit for about a week and then started doing it again. I checked all of the wires and they all seemed tight. Could the new plug wires go bad after a week of use? I'm questioning this because it would have to be more than one plug wire go bad at once.

Thanks for any advice.

First step with a miss fire code is to find out what cylinder is missing.
Have you got good hot blue spark to all cylinders.
Can you find the miss by shorting out each cylinder?
What kind of plugs did you feed it?
Also low fuel pressure and a bad sensor like MAF sensor can cause a miss fire.

Do a search on this site for "intermittant" and look for a thread titled "Intermittant miss at light loads". You'll find a lot of info there. The miss you are experiencing seems to be so common on the Blazer that it's almost scary.

I bought a fuel pressure tester and this is what I found out.

1. Key on\engine off and fuel pump running 62-64 lbs.
2. Pump stops\engine off 55 lbs.
3. Pressure essentially stayed there for 10 min....maybe dropped 2 lbs.
4. 54 lbs idling.
5. jumps up to 60 lbs reving a little.

For what it's worth..... I put Autolite Platinum AP605 plugs in it.
Also tested the MAF sensor with a different one that worked and no change.

Your fuel pressure is low.
Fuel pressure specs is 60/66
You need 62 to 64 engine running.
Even a couple of lbs low will give problems.
Have you found the miss fire?
Pitch the auto lights and put the right Ac Delco plugs in there.

Here is the fuel pressure guide lines.
Check cold start fuel pressure.
Check engine running fuel pressure.
Check engine running on the road fuel pressure.
Then shut it off and watch for fast leak down.
Pinch off at rubber part or block off return line and check full pressure.
Later V6 and V8 gm engines should have 60-66 fuel pressure.
Cold start should be 64-65 lbs of pressure.
If you do not have full fuel pressure on a cold start the injectors will not squirt fuel.
Do not leave home without it.
Engine running should be 60-66 lbs pressure depending on engine load.
Full pressure with return pinched off should be 75-85 or more.
And if it has a fast leak down after shut off you have a leak.
Post back fuel pressure readings.

If I had a dime for every time I read the words "fuel pressure" on this site I'd be rich. My truck had this exact problem for several years through many, many parts and the fuel pressure was 61lbs at initial two second build up and 57lbs while running. The only thing that fixed it was replacing the timing chain on my vehicle, one other person forced a distributor adjustment (timing change) and another replaced their distributor. If the truck only missfires during it's warm up period and settles down after the warm up, chances are it's NOT the fuel pump. I've worked on planes from small "bug smashers" to wide bodies and one thing I've known for a long, long time; the engineers that write the troubleshooting guides rarely ever work on the equipment. If I used the troubleshooting "GUIDES" as a bible, I would have changed A LOT of parts that were perfectly fine. Some common sense can go a long way when it comes to troubleshooting. Read the thread I referenced, hopefully you'll get some ideas. So far, no-one has been able to pinpoint this problem.

All of the previous checks were done when the engine was warm. I did check the pressure when the engine was cold this morning and got essentially the same results. The pressure stayed around 60 lbs while reving the engine a little, but it missed a lot more than when it was warm. The one thing I did look at was the fuel pressure needle, and it did not change as the motor was missing. The question I have is why does this motor run better warm at 60 lbs than it does cold at 60 lbs?

I'll do more testing this weekend.

Other things to note.....

1. This motor has always started up great and idles like a new engine. But for the first 2 miles of driving it misses really bad but then decreases substantially as you drive it. Misfire is less noticeable when the motor has been driven 15-20 miles. Outside temp seems to affect it too. Winter colder temps makes the motor's miss more noticeable even when warm.

2. Should also mention that it does have 190000 miles on it.

3. Fuel gauge does not work from Empty to 3/4. Engine was misfiring before I had this problem.

4. I've had this misfiring problem with the truck for almost a year.

1996LT,

What can I do to check the distributor for any problems.

I really can't give you a definate answer. There are soooo many things that seem to affect this engine. The miss seems to show up more during the warm up time. Once the O2 sensors are warmed up and the computer can adjust the timing for normal operation it seems like the miss goes away. I chased the problem for a few years until a one year old distributor cap shorted out, left my family and I stuck on I95 and I thought the timing chain had jumped because the engine would compression lock during start attempts. After replacing the timing chain, then the cap the missing and hesitations went away. It seems to be a guessing game with this engine. I really don't think your problem is with the fuel pump. My truck had 192,000 on it when I traded it in. No matter how much I did for it, something else would break. I just got tired of feeling like the truck was a full time job. I racked my brain over the miss. Some times it did it when it rained, sometimes when it was cold out, sometimes when it was humid. After a few years I got tired of trying to track it. If you decide to keep it, good luck. I don't know about the 98, but to change the timing chain on the '96 the engine has to be pulled or you'll never get the timing chain cover / oil pan seal to seat properly. One other person replaced the distributor due to some kind of bushing wearing out. Another was able to break some kind of tab and turn the distributor slightly like the older cars use to have done to adjust the timing.

LOL
'96LT

1996LT,

What can I do to check the distributor for any problems.

First step is to get it on a engine capable scanner and check camshaft retard setting. It should be as close as you can get it to - or + 0 degrees as you can get it.
Read out all sensor readings.
Bad sensor readings? MAF,coolant sensor.02 sensors,crank or cam sensor?
A good scanner will show you what miss fires you have past or present and how many miss fires on each cylinder.
And Pull the cap and rotor and have a look at them.
If it needs cap and rotor go AC Delco.
Check dist shaft bushing for excessive wear.
Your fuel pressure is low and can cause engine miss fires.
But do the basics on miss fires.
AC Delco plugs and good wires and cap and rotor.
You need to find out what cylinder or cylinders are miss firing.
Or if it is a ramdom miss fire.
Can you short out he cylinders and actually find the miss?

I really can't give you a definate answer. There are soooo many things that seem to affect this engine. The miss seems to show up more during the warm up time. Once the O2 sensors are warmed up and the computer can adjust the timing for normal operation it seems like the miss goes away. I chased the problem for a few years until a one year old distributor cap shorted out, left my family and I stuck on I95 and I thought the timing chain had jumped because the engine would compression lock during start attempts. After replacing the timing chain, then the cap the missing and hesitations went away. It seems to be a guessing game with this engine. I really don't think your problem is with the fuel pump. My truck had 192,000 on it when I traded it in. No matter how much I did for it, something else would break. I just got tired of feeling like the truck was a full time job. I racked my brain over the miss. Some times it did it when it rained, sometimes when it was cold out, sometimes when it was humid. After a few years I got tired of trying to track it. If you decide to keep it, good luck. I don't know about the 98, but to change the timing chain on the '96 the engine has to be pulled or you'll never get the timing chain cover / oil pan seal to seat properly. One other person replaced the distributor due to some kind of bushing wearing out. Another was able to break some kind of tab and turn the distributor slightly like the older cars use to have done to adjust the timing.

LOL
'96LT

Turning the dist has no affect or chang on engine timing.
The engine timing is set off of the crankshaft sensor.
MT

Turning the dist has no affect or chang on engine timing.
The engine timing is set off of the crankshaft sensor.
MT

I'm one who got angry, broke the tab on the distributor pin clamp and rotated the body to align the spark with the post more correctly. I'm pretty sure mine is th timing chain, dist gears, ect. and agree that checking the cam/crank relationship is first step.

Mine was a 95, the 98's are the same? I rememeber it being called "high voltage switch" or something like that. (wires leave the cap sideways)

Turning the dist has no affect or chang on engine timing.
The engine timing is set off of the crankshaft sensor.
MT

If turning the distributor has no effect, then why won't the engine run if you move the distributor as little as one tooth forward or one tooth backward? If it had no effect then you could just throw the distributor back into it's hole and drive on. The firing of the cylinders HAS to be TIMED with fuel delivery and stroke in order to run, yes?

If turning the distributor has no effect, then why won't the engine run if you move the distributor as little as one tooth forward or one tooth backward? If it had no effect then you could just throw the distributor back into it's hole and drive on. The firing of the cylinders HAS to be TIMED with fuel delivery and stroke in order to run, yes?

On the 98's there is a marking on the distributor cap (number 6) that the rotor should point to when the #1 cylinder is top dead center of compression stroke. Then I think the cam and crankshaft position sensors adjust the timing automatically(No timing light needed). So if you adjust the dist cap a little from the marking on the cap, the sensors should compensate for that adjustment.

If turning the distributor has no effect, then why won't the engine run if you move the distributor as little as one tooth forward or one tooth backward? If it had no effect then you could just throw the distributor back into it's hole and drive on. The firing of the cylinders HAS to be TIMED with fuel delivery and stroke in order to run, yes?


Read my lips.:grinyes:
Turning the dist has no affect on the engine [timing].
You can turn it all day and not change the timing or fuel delivery.
The crankshaft sensor sets base timing.
It is timed off of the crank shaft sensor.

The dist only delivers the spark from the coil to the cylinders buy turning the rotor to the right cylinder at.
Turning the dist centers the rotor to fire dead on with the plug wire terminal.

On the 98's there is a marking on the distributor cap (number 6) that the rotor should point to when the #1 cylinder is top dead center of compression stroke. Then I think the cam and crankshaft position sensors adjust the timing automatically(No timing light needed). So if you adjust the dist cap a little from the marking on the cap, the sensors should compensate for that adjustment.


Here is the markings on dist to set it for installation.
http://files.automotiveforums.com/gallery/showphoto.php?photo=51302

But the PCM/VCM will not adjust anything if you turn the dist.

The dist only has a camshaft sensor in it to let the computer know where the camshaft retard setting is.
In - or + degrees.

The camshaft retard setting set as close to 0 - or + degrees centers the dist rotor to fire dead on or straight at the plug wire terminal.

If it is to far off you will get a code for it being off and if to far off you may get miss fires.

Also if it is to far off you will get a large spark gap which can result in high voltage spark that can damage plugs and engine from to hot of spark.
MT

Read my lips.:grinyes:
Turning the dist has no affect on the engine [timing].
You can turn it all day and not change the timing or fuel delivery.
The crankshaft sensor sets base timing.
It is timed off of the crank shaft sensor.

The dist only delivers the spark from the coil to the cylinders buy turning the rotor to the right cylinder at.
Turning the dist centers the rotor to fire dead on with the plug wire terminal.

Read my lips? Get real. Your words, "Turning the dist centers the rotor to fire dead on with the plug wire terminal." You couldn't be saying that turning the distributor positions the rotor in such a way that the sparked is sent to the plug wire terminal at a specific time in order to coincide with the compression stroke and fuel delivery. Saying that would lead me to believe that turning the distributor would change when the spark is sent to the plug wire terminal. But that couldn't be since turning the distributor has no effect on when the spark is sent to the plug.
Since I obviously don't understand this and what you say must be right, if I put a timing light on my vehicle and turn the distributor while it is running, the timing marks under the gun will not be affected? Gee, I thought that a part of timing the engine ment you were timing the spark to be delivered at a specific time. Silly me.

MT is correct, the distributor is only that. It merely distibutes the timed spark to the correct terminal. It does not change the timing, it only changes the rotor to post relationship.

Think of it as loosening just the cap itself and swinging it only.

YIPPPPEEEE. IT'S FIXED!!!!

Well....I called the junk yard today to see if they had a distributor to put in my Blazer. They did....$40 bucks. So I thought, what the heck, I'll give it a try. Put her in and bam.....runs like a champ. But I'm thinking it may not have been the distributor that was bad, but the cam sensor. Not 100% sure but I didn't really see anything wrong with the distributor itself. Plus.....I think what was happening is the cam sensor needed to be warm in order to work correctly and it took time for the heat to reach the distributor area so it would work correctly. Just my guess....so I'm sticking to it.:grinyes:

I want to thank the people on this site. You guys sure know your stuff and it helped me solve the problem.

Thanks again!!