98 1500 5.7L Random Misfires

[Beaner81]

I have a 98 Chev 1500 5.7L with 215,000 miles and a few minor issues. I noticed while towing and climing a hill if I gas it up a hill, once I get over the crest it will start misfiring like the plugs are loading up and misfiring. If I dont let off the gas and let it clear out it will trip a random cylendar misfire code. I put in new delco plugs, wires, cap and rotor, still does it. Could it be a bad cap?
Next my dash security light comes on periodically but doesn't seem to affect anything but on occasion I will start my truck and it will die out right away, it will do this over and over untill I leave it alone for 5 or 10 minuites then try again and it will start right up. Maybe bad ignition?
Thanks

[Airjer_]

The first thing I would do is see what the missfire history is for each cylinder. You will need a more advanced scan tool for this but I think there is a possibility that a coil may be getting week after a hot soak (pulling a trailer up a hill willl work everything a lot harder).

Problem 2, Have you tried using any of the other keys you have to see if the problem exists with those as well. If it doesn't than throw away the key that gic=ves the problem and have a new one made. This is a very cheap and easy first step to figuring these out.

[Beaner81]

My neighbor is a mechanic and he brough his scan tool from work and told me it was a random misfire and not just on one particular cylendar. Are you leaning towards a new distributor as being the fix?

I will try using my spare key for a while. Thanks

[Airjer_]

I was thinking this had a coil over ignition. Now that I realise it is not than the possibility exists that it is still an ignition issue but likely not the coil.

The good scan tool should have been able to view some sort of missfire history data (im pretty sure). That is usually the first thing I look for when I see a random missfire code. Typically there will be a few cylinders with more misfires than the rest. Since those are the "weakest link" I try to focus my attention on those.

It is very possible that a simple tune or a major tune-up will solve your issue. Inspecting the plugs would be a good start. If worn out than replace and see what happens.

Wetting down the wires will help find any leaks. If they arc they are bad. Same with the cap.

Inspecting the cap and rotor can't hurt either. You'll be looking for a lot of corrosion in the cap and black arc burns around the rotor.

[j cAT]

Originally Posted by Beaner81

I have a 98 Chev 1500 5.7L with 215,000 miles and a few minor issues. I noticed while towing and climing a hill if I gas it up a hill, once I get over the crest it will start misfiring like the plugs are loading up and misfiring. If I dont let off the gas and let it clear out it will trip a random cylendar misfire code. I put in new delco plugs, wires, cap and rotor, still does it. Could it be a bad cap?
Next my dash security light comes on periodically but doesn't seem to affect anything but on occasion I will start my truck and it will die out right away, it will do this over and over untill I leave it alone for 5 or 10 minuites then try again and it will start right up. Maybe bad ignition?
Thanks

I would first measure the plug wire resistance...If the resistance is too high the spark may be getting knocked down due to the higher compression created under a heavy load...

could also be coil and corrosion of the coil grounds and connections...


low spark voltage or high plug wire resistance..

when towing do not use 4th gear ...

[777stickman]

OK: So you've replaced the plugs, wires, cap & rotor (all AC-Delco?) and the problem continues.

So let's look at some other stuff that can cause this.

Other electrical stuff involved are the CMP, CKP, ICM and wiring.

Fuel delivery stuff are pump, filter, injectors, air filter, MAF, IAC, etc.

As far as your mechanic neighbor and his scan tool? Only a good high end scan tool can access "current" and "history" misfires. "History" misfires can tell you if all cylinders are doing it or just some.

Question. Are there any other "current" or "history" codes stored in the VCM? Also, in the future, we need the exact DTC code number, not just a description!

More stuff to think about!!

[Schrade]

Originally Posted by j cAT

I would first measure the plug wire resistance...If the resistance is too high .

A SLIGHTLY high resistance wire had me chasin' ghosts ALL summer in my '94 vette. Half a dozen false codes. Saw "Check wires resistance" one day, in the FSM. Unreal.

[MT-2500]

Originally Posted by Beaner81

I have a 98 Chev 1500 5.7L with 215,000 miles and a few minor issues. I noticed while towing and claiming a hill if I gas it up a hill, once I get over the crest it will start misfiring like the plugs are loading up and misfiring. If I don't let off the gas and let it clear out it will trip a random cylinder misfire code. I put in new delco plugs, wires, cap and rotor, still does it. Could it be a bad cap?
Next my dash security light comes on periodically but doesn't seem to affect anything but on occasion I will start my truck and it will die out right away, it will do this over and over until I leave it alone for 5 or 10 minutes then try again and it will start right up. Maybe bad ignition?
Thanks

As said you need to get it on a high end high dollar engine capable scanner.
If misfire history shows only 1 or 2 cylinders misfiring go for secondary ign problems.
If random misfiring on all cylinders.
Get in the scanner and check camshaft retard settings and fuel trims.
Check for low or high fuel trims on both banks.
If fuel trims are off and same on both banks.
Check for proper fuel pressure.
Low fuel pressure and or a MAF sensor or camshaft retard not set right will give random misfires on all cylinders.
Make sure camshaft retard setting is at or within 1-2 degrees of - or + degrees,
Try it with MAF sensor unplugged and see if it still misfires.
For fuel pump and pressure check.



The only true test on a fuel pump is to check the direct fuel pressure from fuel pump.
A quick test is to block off the return line and see if fuel pressure comes up to 75-85 lbs.
But do not run the pump at full pressure very long.
Also when testing fuel pump and pressure you need to tape a gauge to outside windshield or outside mirror and drive it on the road for 20 -30 minutes
until the pump gets has run a while to check for a pump fading out after hot.

Hard to start cold and fuel pressure testing guide line.
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.
If pump has full pressure with return line blocked and low pressure without it blocked most usually the fuel pressure regulator is leaking or not holding 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 in system.
Post back fuel pressure readings.

[j cAT]

Originally Posted by cbec1999

A SLIGHTLY high resistance wire had me chasin' ghosts ALL summer in my '94 vette. Half a dozen false codes. Saw "Check wires resistance" one day, in the FSM. Unreal.

many years ago I learned the hard way on plug wires....NEW MEANS NOTHING ...all wires are measured by me....always...new wires must be close to the oem spec...on resistance...

[Schrade]

Originally Posted by j cAT

many years ago I learned the hard way on plug wires....NEW MEANS NOTHING ...all wires are measured by me....always...new wires must be close to the oem spec...on resistance...

I'd like to hear that story, because mine was runnin' tip top under all conditions, including 30+ mpg's highway. I had NO reason to replace wires, and had done THOROUGH diagnostics...

[Beaner81]

Thanks for all the info. It seems most replies are related to fuel pressure which is interesting because I replaced the fuel pump about 14 months ago and for the last 8 months I have been getting a fuel evap code that comes and goes. I was thinking unrelated but now it seems to have a common thread. Where is the most likely place to have a leak that doesn't leave fuel on the ground under the truck but still trips the evap code. I did try a new fuel cap just for a quick fix (its never that easy). I can also say that the misfiring seems to happen more frequently in damp weather during or after rain.

Anybody have any ideas on the security light and engine starting problems I listed in the first post?

[j cAT]

Originally Posted by cbec1999

I'd like to hear that story, because mine was runnin' tip top under all conditions, including 30+ mpg's highway. I had NO reason to replace wires, and had done THOROUGH diagnostics...

since you have a diagnostic computer to chart out all of these tests on this, then you are most assured to have no defects..


I am not that well off ...I wish I had that diagnostic equiptment..

[MT-2500]

Originally Posted by Beaner81

Thanks for all the info. It seems most replies are related to fuel pressure which is interesting because I replaced the fuel pump about 14 months ago and for the last 8 months I have been getting a fuel evap code that comes and goes. I was thinking unrelated but now it seems to have a common thread. Where is the most likely place to have a leak that doesn't leave fuel on the ground under the truck but still trips the evap code. I did try a new fuel cap just for a quick fix (its never that easy). I can also say that the misfiring seems to happen more frequently in damp weather during or after rain.

Anybody have any ideas on the security light and engine starting problems I listed in the first post?

They are three seperate problems.
The evap code is fuel tank not holding engine vacuum.
Not a fuel leak.
The security light anti theft shuts off injector pulse.
If it is a problem it will set a body code.

As said misfire can be caused by several things.
Proper testing is in order to find misfire.
If more in damp weather check wires and dist cap rotor and for cracked plug.

What brand of fuel pump did you put in it?
If Airtex you may be in a world of hurt.
Get a fuel pressure gauge and test it out as outlined in upper post.

[j cAT]

Originally Posted by Beaner81

Thanks for all the info. It seems most replies are related to fuel pressure which is interesting because I replaced the fuel pump about 14 months ago and for the last 8 months I have been getting a fuel evap code that comes and goes. I was thinking unrelated but now it seems to have a common thread. Where is the most likely place to have a leak that doesn't leave fuel on the ground under the truck but still trips the evap code. I did try a new fuel cap just for a quick fix (its never that easy). I can also say that the misfiring seems to happen more frequently in damp weather during or after rain.

Anybody have any ideas on the security light and engine starting problems I listed in the first post?

This is only concerning the fuel evap problem......this is a problem that is very common and widspead in the rust belt....Fuel filler neck has a rust hole in it... this I am all to familiar with ...this will get you this failure code..


If this is found the cost is 211.00 US....use of metal mender is used most often rather than replacing the filler as this code will trip with a pin hole...

[Beaner81]

Ok guys, I got a better scan tool and it turns out its not a random misfire, I had 906 misfires on cylendar 8 and 20 on cylendar 4. I am thinking about continuing from here with a compression test on that cylendar. Considering that I had this same issue before I changed the plugs, wires and cap I am temporarily ruling those out as problems. Any other suggestions?