1997 C1500, 4.3L random misfire, at constant speed

[bigrobs1973]

Hello,
I have a problem with my truck that my local mechanic cannot seem to figure out. Maybe someone on this forum could help.
I have a 1997 Chevy truck 1500 series. It has a 4.3L engine, 4L60 transmission. The truck starts good and idles good. But, it misses at a constant speed from (take off to level out), It does not misfire (or jerk or shake) while it is under a load. My mechanic checked the ECM for codes and found a P300 random cylinder misfire. I replaced the cap, rotor, wires, plugs and pick up coil (all AC Delco parts). No change in the performance what so ever. Has anyone had or know of anyone having a problem like this? Please, any help would be great.

[MT-2500]

Originally Posted by bigrobs1973

Hello,
I have a problem with my truck that my local mechanic cannot seem to figure out. Maybe someone on this forum could help.
I have a 1997 Chevy truck 1500 series. It has a 4.3L engine, 4L60 transmission. The truck starts good and idles good. But, it misses at a constant speed from (take off to level out), It does not misfire (or jerk or shake) while it is under a load. My mechanic checked the ECM for codes and found a P300 random cylinder misfire. I replaced the cap, rotor, wires, plugs and pick up coil (all AC Delco parts). No change in the performance what so ever. Has anyone had or know of anyone having a problem like this? Please, any help would be great.

Good ac delco cap and plugs?
Low fuel pressure or ign problems can give misfires.
Needs to be put on a good engine capable scanner and read out what and how many times the cylinders are misfiring.
Also check camshaft retard setting in scanner.

Make sure fuel pressure is up to par.

[bigrobs1973]

Replaced cap and plugs with genuine Delco parts. I took the truck to another mechanic and found that the fuel pressure is too low, he is going to check out furhter and let me know by Saturday. Thanks for the help.

[gandl2123]

I have a 97 C1500 4.3L Base truck. I love the truck and she starts instantly and idles perfectly...Put it in gear and she takes off fine under load and then just before she shifts (few seconds before) she stumbles and misfires and I get a P301 code....then she shifts to next gear and smooths right on out until next shift.....

1.) My book says 60-66psi. Is that right?
2.) My truck is giving me 56-58psi at the rail.

Before I got that I was at 52-54 and the truck was backfiring and acting/running terrible. Using WAY too much fuel. So, I took the plenum off and changed the fuel pressure regulator (a used one, I know that was stupid). I noticed an immediate improvement and pressure at 58 or so.

Now I'm stuck with a very minor (as explained) mis and studder near the shift points. Could it be that I need to put a NEW regulator on it to get the pumps 60-66 psi capabilities or do I need to put a pump in it?

I would love to check the pressure short of the regulator but don't know how. I really think it may be regulator because the pump with key on is INSTANT to 58. I mean it takes 1-2 seconds and it is solid 58 and holds for 20 min with key off.

Hope this bit of information from me helps someone and hope someone can verify what they think I should do in my situation.

Had the spider thing off and injector tips / poppets looked very clean (sprayed off with carb cleaner anyway).

Thanks for you help.

Mr. Tripp
Augusta, GA

[MT-2500]

Originally Posted by gandl2123

I have a 97 C1500 4.3L Base truck. I love the truck and she starts instantly and idles perfectly...Put it in gear and she takes off fine under load and then just before she shifts (few seconds before) she stumbles and misfires and I get a P301 code....then she shifts to next gear and smooths right on out until next shift.....

1.) My book says 60-66psi. Is that right?
2.) My truck is giving me 56-58psi at the rail.

Before I got that I was at 52-54 and the truck was backfiring and acting/running terrible. Using WAY too much fuel. So, I took the plenum off and changed the fuel pressure regulator (a used one, I know that was stupid). I noticed an immediate improvement and pressure at 58 or so.

Now I'm stuck with a very minor (as explained) mis and studder near the shift points. Could it be that I need to put a NEW regulator on it to get the pumps 60-66 psi capabilities or do I need to put a pump in it?

I would love to check the pressure short of the regulator but don't know how. I really think it may be regulator because the pump with key on is INSTANT to 58. I mean it takes 1-2 seconds and it is solid 58 and holds for 20 min with key off.

Hope this bit of information from me helps someone and hope someone can verify what they think I should do in my situation.

Had the spider thing off and injector tips / poppets looked very clean (sprayed off with carb cleaner anyway).

Thanks for you help.

Mr. Tripp
Augusta, GA

You are welcome and we are glad to help.

You will need afuel pressure gauge that you can hook up to truck and drive on road and watch fuel pressure while driving it.
Like gauge taped to outside windshield or outside mirrow.

Fuel pressure specs are 60/66 on that engine.

If pressure is low block off or plug return line and see if fuel pump pressure comes up to 95/105 lbs pressure.
Just do not run pump at full pressure over a second or 2.

Direct fuel pump pressure has to be up over the regulated regulator specs and also have enought volumn to hold regulated pressure.

Code 301 points to miss on no 1 cylinder.

Make sure good compression and plugs and cap and rotor and wires on that cylinder.

Beside low fuel pressure The camshaft retard setting and secandary ign and even MAF sensor and other sensors can create a miss fire.
A good place to start is to.
Work on getting the fuel pressure up to specs.



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.

Guide lines on checking fuel pressure and pump test.

Hard to start cold or hot or low fuel pressure and fuel pressure testing 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.
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.

[gandl2123]

I did go up under the truck and put a pair of vise-grips on the return line. Got up and started the truck...it just sat there and purred like a kitten....fuel pressure rose to maybe 62 and that was it. It did not go way high and start spitting out black smoke or anything.....I was left thinking I had some kind of leak down....fuel was not going aywhere that I could see.

Is it just that the pump is not able to pump more than that? IDK

Key on pressure: 56 in less then 2 seconds.
Turn off: Pressure holds for 20 min or so without dropping much at all.

Start engine: Pressure may be 58 if that.

Pinch of return line: Pressure may go to 60 to 62 and that is it.

I'm beginning to think pump. Truck has 214,000 miles on it and purrs like a kitten except for the hesitation/stutter.

[jdmccright]

Ayup, it's the pump. A pinched return line should shoot the pressure high as mentioned.

Spend the extra money and get a Delco pump. You'll save yourself the pain of replacing it a year from now if you use a parts store brand such as Airtex.

Good luck!

[gandl2123]

Thanks...I'll get a good pump. Just can see why people choose the lesser one...the cost of the pump is ridiculous.

Tks

I will try to remember to post back after I finish the install and tell if that was the solution completely.

[gandl2123]

Hey everyone!

Can you tell me how to find the fuel pump alone and not the whole module?

I am allergic to paying too much for a module I don't need. I just want the pump and strainer and I can fix the rest.

Also, we have a "Pull-A-Part" major chain used auto parts yard and I may be able to get lucky and find a low mileage vehicle with same pump/module. I know many would not recommend this but when you are on a budget you do funny things and I have been very lucky with used parts in my life.

What vehicles would I be looking for that would have the same pump? Many have the same pump but the module is different and therefore they say it won't interchange but it is the same pump.....any info?

Tks

[MT-2500]

The AC- Delco pump is usually known as and goes by Delphi fuel pump.

Cost about 25$ more than junk Airtex and aftermarket pumps.
And a lot of places you have to special order them.
But usually only overnight.
Carquest is about the only place that carries the Delphi pump in stock.
They have dropped the other brands like a hot spud.
Some parts places only stock and pusk the junk ones.
If so get on ther case and tell the Delphi is the only one you want.

Other things beside the pump can give also give low fuel pressure.
Like wiring and plug ins and grounds and voltage to pump and fuel pump relay to pump and fuel filter plugged.
It is always best to go threw the full fuel pressure test before and after replacing the pump.

[MT-2500]

The AC- Delco pump is usually known as and goes by Delphi fuel pump.

Cost about 25$ more than junk Airtex and aftermarket pumps.
And a lot of places you have to special order them.
But usually only overnight.
Carquest is about the only place that carries the Delphi pump in stock.
They have dropped the other brands like a hot spud.
Some parts places only stock and pusk the junk ones.
If so get on ther case and tell the Delphi is the only one you want.

Other things beside the pump can give also give low fuel pressure.
Like wiring and plug ins and grounds and voltage to pump and fuel pump relay to pump and fuel filter plugged.
It is always best to go threw the full fuel pressure test before and after replacing the pump.

[MT-2500]

Originally Posted by gandl2123

Hey everyone!

Can you tell me how to find the fuel pump alone and not the whole module?

I am allergic to paying too much for a module I don't need. I just want the pump and strainer and I can fix the rest.

Also, we have a "Pull-A-Part" major chain used auto parts yard and I may be able to get lucky and find a low mileage vehicle with same pump/module. I know many would not recommend this but when you are on a budget you do funny things and I have been very lucky with used parts in my life.

What vehicles would I be looking for that would have the same pump? Many have the same pump but the module is different and therefore they say it won't interchange but it is the same pump.....any info?

Tks

On a buget I can understand looking for a way to save a buck.
But.

There is few short cuts or ways to save money on a good fuel pump.
Outside of checking around/pricing the delphi parts.
Some places will give a better price on them.
After pricing around for best deal.
Best to just back up to the parts counter where they can bite you in the rear and tell them you need a New Delphi fuel pump module.

Like My uncle always said the good stuff only hurts one time.

Cheap parts or bone yard stuff can bite you in the rear many times.


Fuel pump alone are very hard to replace in the module.
Due to the plastic lines connecting the pump to fuel lines and wiring confirration.
And pulling one from a salvage yard there is a slim chance of getting a real good pump.

Bad fuel pump cold even be the reason it is in the bone yard.

If you are going to bone yard it i would try to get the complete tank that will fit you truck.

Good Luck and let us know how it goes.

[gandl2123]

Thanks for the information. I have located a local parts house that sells CARTER fuel pumps. What do you think about this name? IMHO it is a better brand then the knockoffs but not sure how it stands up to Delphi which is a part of GM right? Tks

[MT-2500]

Originally Posted by gandl2123

Thanks for the information. I have located a local parts house that sells CARTER fuel pumps. What do you think about this name? IMHO it is a better brand then the knockoffs but not sure how it stands up to Delphi which is a part of GM right? Tks

You are welcome.
Carter is a very old brand and maker of many older fuel pumps.
May even be older than I am and that is old.
I have used many Carter older type fuel pumps over the last 50 years with no problems.

But the newer high pressure fuel injection pumps are a different breed.

Airtex and other brands used to make good fuel pumps to but went down hill.
Some even sold out there brand to make and supply cheaper fuel pumps.

Yes Delphi has replaced AC-delco and most GM fuel pumps are made and branded by Delphi.

Delphi also makes a lot of other electrical parts for GM OEM.

Not sure but I would supect GM has stock in Delphi or GM big shots are sleeping in bed with Delphi big shots.

As far as Carter Brand Pumps ask your parts place how many Carter pumps High pressure for GM they have sold that handles them and what the fail rate is on them.

How does the Carter Price tag stack up with Delphi pumps?
Do they sell only have Carter or other brands?

[gandl2123]

White Brothers Auto Parts here In Augusta, GA has the pump for around 158.00 but they said, if it has GHPV or INOP...all types of different letters on the top of the pump to get the right one....

they say it depends on the pump it requires and that I should call the dealer and see which one it calls for.....GMAB!

Sooooo, guess I have to take it off and hope that after 214,000 miles that it has the original pump and has the letters legible so I can tell them what pump I need.

Tks