Low engine rpms, intermittent stalling.

[ken_man_1]

Hey, I've got a 99 5.7L, auto trans, air, 4x4 - 81k. I did a tune on it in December with all delco parts. New dist, rotor, wires, plugs. Replaced egr last year, along with tps and iac. The engine lopes once in a while, like around 700rpms down to like 400 rpms in gear. Sometimes it will drop down to as low as 200 rpms. Sometimes it will die is you shift from reverse to drive. I thought it was the iac valve, but now I'm stumped. No engine light. Fuel pressure is at about 59 while idling.

Also noted that it misfires between about 850rpms to about 2200 or so while accelerating. I have replaced the MAF with a known good working unit, so I'm ruling that out as well. Have sprayed carb cleaner around the intake as well as the tubing that goes to it. No change in RPM. No codes.

Any ideas would be greatly appreciated.

[danielsatur]

1) Clean IAC, MAF sensor Air filter, and throttle body.

[ken_man_1]

Replaced IAC, swapped maf with good known working unit, and cleaned throttle body twice. No difference.

[j cAT]

Originally Posted by ken_man_1

Replaced IAC, swapped maf with good known working unit, and cleaned throttle body twice. No difference.

If as YOU SAY used carb cleaner on the throttle body/IAC this then is damaged. the coatings/wiring gets damaged. you must use throttle body cleaner only. carb cleaner is for metal only. this product eats at wires/rubber/plastic parts.

the other item to check with poor idle control would be the PCV. the hoses and valve should be replaced if original.[CRANKCASE VENT SYSTEM LEAKS]

[danielsatur]

Pinpoint any vacuum leaks with a smoke test, and focus on EGR, PCV, and air intake systems.

Note - When the engine gets caught in a bad lope, turn her off and do a restart!

Is the Incoming Air Temp (IAT) sensor housed in the MAF?

If not, clean the IAT sensor and connector with MAF sensor cleaner.

[777stickman]

One other thing that hasn't been mentioned is the Coolant Temp Sensor for the VCM located on the intake manifold.

[ken_man_1]

That coolant temp sensor was replaced with a new one, and also tried with a known good working unit - it didn't change anything. I'm at the point of swapping distributors with a good known unit, and see what happens after that. I have an identical truck year/model, so going back and forth between them is ok, but I HATE the distributor deal. Thanks for all the posts. I'll let you know what I find in the next days ahead.

[j cAT]

you have to get the engine codes which should have been mentioned at the start of this.

the carb cleaner useage could have caused damage. I also would do a vacuum test and find any leaks. the IAC throttle body passageways could be restricted. use THROTTLE BODY CLEANER with A SMALL BRUSH.

the other item could be the ICM.

whats the fuel pressure ? do the fuel pressure test proceedure ...........

[ken_man_1]

Originally Posted by j cAT

you have to get the engine codes which should have been mentioned at the start of this.

the carb cleaner useage could have caused damage. I also would do a vacuum test and find any leaks. the IAC throttle body passageways could be restricted. use THROTTLE BODY CLEANER with A SMALL BRUSH.

the other item could be the ICM.

whats the fuel pressure ? do the fuel pressure test proceedure ...........

No codes. I did use throttle body cleaner. The carb cleaner was sprayed only around the base of the upper plenum - not into the intake.

The fuel pressure is at 62 eoko, and 59 idling.

One other thing to note is that I had a test done on the injector balance, and every single injector showed same drop - 8psi.

The beginning of the post, I stated that I replaced the distributor - but I meant distributor cap, not the whole thing.

To restate what I replaced thus far - dist cap, rotor, plug wires, plugs (needed routine maint anyhow) - iac, coolant temp sensor, and swapped iat and maf sensor with good known working unit.

I will look at the pvc system and see how that looks as well.

Thanks again for the posts and thoughts.

[j cAT]

with missfires you should get DTC codes.

with the idle rpm drop this would be the fuel pressure droping out. could be fuel pump / voltage components to the pump. relay/wiring/loose fuse socket.

run the vehicle with a longer hose to see if the fuel pressure stays up.

ignition on engine off 62-65 psi. engine running should be above 55psi.

ICM also can cause rpm/missfires. these can be tested at most autozone stores with the wells tester.

[ken_man_1]

I got busy today looking at the cap/rotor/wiring. Took the cap off, grabbed the rotor and it had quite a bit of movement. Took dist out and about crapped when I saw the gear. I replaced it 2 1/2 years ago and only 10k on it. It's worn out very unevenly, more toward the bottom. I've replaced these gears before on different trucks, but never twice on 1 truck. Time for a new distributor I guess.

[Schurkey]

Originally Posted by ken_man_1

about crapped when I saw the gear. I replaced it 2 1/2 years ago and only 10k on it. It's worn out very unevenly, more toward the bottom. I've replaced these gears before on different trucks, but never twice on 1 truck. Time for a new distributor I guess.

First Guess: Gear on camshaft is wiped-out, too.

[j cAT]

Originally Posted by ken_man_1

I got busy today looking at the cap/rotor/wiring. Took the cap off, grabbed the rotor and it had quite a bit of movement. Took dist out and about crapped when I saw the gear. I replaced it 2 1/2 years ago and only 10k on it. It's worn out very unevenly, more toward the bottom. I've replaced these gears before on different trucks, but never twice on 1 truck. Time for a new distributor I guess.

many say to use only the OEM GM gear. this is made so that the cam gear does not get worn. some of these aftermarket gears can wear down the cam gear. I would replace the distributor because it may be draging causing the gear to wear rapidly.

[ken_man_1]

Originally Posted by j cAT

many say to use only the OEM GM gear. this is made so that the cam gear does not get worn. some of these aftermarket gears can wear down the cam gear. I would replace the distributor because it may be draging causing the gear to wear rapidly.

Yep, used oem gears only. But in this case I replaced the whole thing.

Now, it helped, but not a whole lot. It no longer misfires, but it does still drop down the rpm's to about 250-300 when shifting from drive to neutral. I'm going to have the cam retard checked to see if that may be an issue - but still no engine light or codes. I do have the cheapo scanner that tells codes . More to come after I get the cam retard set.

The cam gear looks just fine...

[ken_man_1]

Just as a side thought, if the distributor is dropped in and it's not in the exact same spot as the other one, won't that throw a code, too? It's still dropping down to 250 going from drive to neutral, or reverse to neutral. Driving me nuts...