99 4.3 Rough Idle, Low Temp, Bad Gas Mileage

[dr_lee29]

I'm leaning towards the airtex pump for $120 less.

[MT-2500]

Leaning that way can get you in a world of hurt.

For the last several years airtex/aftermarket fuel pumps have been junk.
In the link I posted there garage cost 323$ on AC-Delco/delphi dealer pump.

O'Rielly/ first call Auto parts.
Airtex junk 288.40$ plu s wiring harness CHANGE OVER 20-30$ dollars MORE.
Ac-Deco/delphi 323.80$ should not need wiring change over.

Auto zone also has good price on the ac-delco pumps.

As general a rule Derlphi runs about 25$ more than airtex/aftermarket.

Check qround ask around.
Good luck

[dr_lee29]

I saw the price in the post but I called Napa and that was their price, they didn't have any other pump. Autozone said they couldn't even order Delco.
By the way on the post that you linked to, the problem wasn't a bad pump it was a bad regulator maybe two bad regulators.
I'll keep looking for a delco pump but I can't see paying $100 more for another pump. The truck has 150K on it now, will it out live the pump?

[tempfixit]

Try Carquest for Delco pump, seems to me they switched from Airtex to Delco.

[dr_lee29]

Still hunting.
Found a Delphi at CarQuest for $395.35 and a Carter at PepBoys for $249.36. I called the dealership parts and services and they said they couldn't find the part with out my VIN!! I told them that it was an "X" engine but they insisted on the VIN before giving me a part and price. Are these pumps really that specific?

[MT-2500]

Originally Posted by dr_lee29

Still hunting.
Found a Delphi at CarQuest for $395.35 and a Carter at PepBoys for $249.36. I called the dealership parts and services and they said they couldn't find the part with out my VIN!! I told them that it was an "X" engine but they insisted on the VIN before giving me a part and price. Are these pumps really that specific?

Good luck
If you can find the right place ac/delphi is only 25 dollars over the airtex or cheap aftermarket pumps.

[dr_lee29]

New pump, same results. Fuel pressure is exactly the same. Idle is exactly the same. The truck has plenty of power, I gave it heck on the country roads tonight and it can get up and go but that has always been true.

[MT-2500]

Originally Posted by dr_lee29

New pump, same results. Fuel pressure is exactly the same. Idle is exactly the same. The truck has plenty of power, I gave it heck on the country roads tonight and it can get up and go but that has always been true.

What kind /brand of pump did you end up with?

I would go threw the fuel pressure test as described to confirm it is a good pump or other problem.

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.

[ericn1300]

If you still have the low operating temps and a miss on #4 only I think the shop was right about an intake gasket leak. Also do a compression check, could also be a head gasket. Either one can cause the low temps and miss fire. Also check for water in the exaust when warmed up and spray some carb cleaner along the intake gasket around #4 to check for leak.

We have a 3.1 with simular symptoms. The dealer told them it was a fuel pump but a new pump, filter and injectors (TBI) did not fix the problem. Turned out to be the intake gasket. Try re-torquing the intake first, you just might get lucky.

[dr_lee29]

OK lets talk more about the gasket problems. I have noticed losing some coolant but I wasn't sure if it was because I keep having to take off the thermostat housing and when I was under the truck today I noticed a drip of water on the bottom of the muffler. The first thing I changed when this whole thing started was the LMG, I'm pretty sure thats fine but I'll try torquing the manifold again. How would I know the heads were leaking?

[ericn1300]

not sure what you mean by the LMG, but if you took off the intake manifold and didn't use a new bolts and gasket kit, well that could be your problem.

go by the book on the torque pattern and specs

small head gasket problems are even harder to diagnose, do a compression check, check for water in the exhaust after a good hot run, and watch the coolant and oil for loss or contamination.

[dr_lee29]

LMG = lower manifold gasket
I did buy new gaskets and put new RTV sealant on the front and back of the manifold. I cleaned the surfaces well also. I did go by the book on torque and pattern (I think it was 18 lb/in if I remember?). I traced the threads in the bolt holes but didn't put new bolts in. I didn't know that was a necessity. Actually, everything is pretty clean under the manifold also, no oil/coolant mix.I really don't know how it can be a lower manifold gasket problem. Thats what the mechanic thinks the problem is. I could be convinced its a vac leak but not on the lower manifold gasket.

Compression on #4 is 130 dry. It fluctuates between 0-50 running. I get similar numbers off of #2 which isn't missing.
The miss goes away after 2300 RPM.

Out of ignorance, what happens if I continue to drive the truck in its present condition? Will I foul the #4 cylinder and have to overhaul?

[MT-2500]

Originally Posted by dr_lee29

LMG = lower manifold gasket
I did buy new gaskets and put new RTV sealant on the front and back of the manifold. I cleaned the surfaces well also. I did go by the book on torque and pattern (I think it was 18 lb/in if I remember?). I traced the threads in the bolt holes but didn't put new bolts in. I didn't know that was a necessity. Actually, everything is pretty clean under the manifold also, no oil/coolant mix.I really don't know how it can be a lower manifold gasket problem. That's what the mechanic thinks the problem is. I could be convinced its a vac leak but not on the lower manifold gasket.

Compression on #4 is 130 dry. It fluctuates between 0-50 running. I get similar numbers off of #2 which isn't missing.
The miss goes away after 2300 RPM.

Out of ignorance, what happens if I continue to drive the truck in its present condition? Will I foul the #4 cylinder and have to overhaul?

Can you explain how it fluctuates 0-50 running.

When you run compression test always hold TB open.
And make sure spark plugs/cylinders have been running dry before checking compression.

A wet or dead cylinder will give a false reading.

On a compression test always look for not over 15 percent variation between cylinders.

What are all of the cylinders reading on a dry compression test?

It is not good to run a engine with a dead cylinder.

[Spectria, L1 MT]

Hello, MT-2500 you do have the patience of JOB.

Question 1)
I would like to know if any of the test results were with a hot engine, you know, at proper operating temp.

My basic knowledge is that the computer will always feed additional fuel to a cold (colder) engine to compensate for poor fuel atomization in the COLD air stream.

If any sensor is telling the computer the engine is cold (colder) it will compensate with additional fuel.

This can cause an uneven idle and poor fuel mileage.

If you can't get the engine up to temp changing out thermostats, try covering the radiator with a blanket until you get at least 195. Then run your tests at temp.

Question 2)
Is a cylinder leak down test appropriate to see if we can determine compression leakage at an intake or exhaust valve (I know this is old school, but so am I)??

This test might reveal a reason for low speed misfire at #4, although I am inclined to believe it is an intake leak at #4.

A slight valve leak will be far more evident at idle and will tend to disappear to speed. It may not show up in a compression test (wet or dry) depending on the engine temp at the time of the compression test.

Engine should be at operating temp (195), pull ALL of the plugs, disable the fuel pump prior to the test. I had a friend who made this test with the pump connected and the ignition working and had to go to the hospital for facial burns.

Question 3)

Did you put in a Dephi pump or some other unit? What were the pressures as described by MT-2500 in his test sequence?

I have found very common problems with every brand of knock off pumps, from poor electrical connections, to low flow and pressure, to leaks in the body, all of which are hard to test for while in the tank. Use the more expensive unit, since you have thrown a ton of money at this vehicle in every other way.

Please post results. Thank You MT-2500. You are the MAN.

[dr_lee29]

No tests have been done with the engine hot. The engine will not get hot and I think this is the real problem. Some how there must be coolant flowing through the engine and bypassing the thermostat, I can't explain it any other way. It seems like I have to address this problem before I can fix anything else. Can a head gasket leak cause low temps?

I did not do the compression tests, so I can't tell you exactly how those tests were done. They were done when I sent the truck to the mechanic he told me the results and I relayed them to you. He said they look normal, I believe him. But he thinks its a lower manifold gasket, I had already changed this and know its in good shape.

I did put a Delphi pump in, I think its ridiculous to pay $125 more for a pump but thats what I did. There were NO delphi pumps in a 50 mi radius for an extra $25. They are always $125 more. I'm still not convinced it was the right thing to do. The pressure is exactly the same as before. After 30 minutes of running the truck (pump is warm, engine is warm not hot) the pressure on the block is 60 psi with key on engine not running. 52 psi running engine, 58 psi at 2500 RPM. Bleed down is normal. Directly off the pump, 95 psi, maybe a little higher but I don't let it run long on this test. Remember this is with a new regulator, filter, pump.

I'm also pretty sure I'm losing coolant, but I've had the thermostat out several times and it could be that the engine hadn't burped when I checked it the first time. I do not see coolant leaking out of the engine.

I just don't believe this is some obscure problem, I actually expected that someone else would have dealt with this before and chimed in by now. But I really appreciate the help I've gotten so far.