Can you guys help me with a 4.3L V6 starting problem?

[argo]

I have a good friend with a 2001 Silverado with a 4.3L V6, 153,XXX miles. It's a basic truck; 2wd shortbed, automatic transmission, A/C, and manual everything else. He recently bought it and it has an unusual starting problem. If you turn the key to start it, sometimes it cranks right up and runs like a top. Other times, when you crank it, it acts like it's starved for fuel. It just doesn't start. However, if you turn the key off, then try to start it again, it will crank right over, no problem, and run like a top. I have spent a great deal of time checking this thing over, but I am lost. I have virtually no experience with GM's CFI/CMFI setup, and I feel like there's something I am missing here.

What I have checked:

I have tested the fuel pump relay, and I have even substituted it with a known good relay for comparison. The relay is OK.

I do not have a pressure gauge that will work with this system, so I do not have fuel pressure readings, but I do know that the system will drive the O2 sensors rich at WOT from 0-100+ MPH according to my Genysis (that was on a long stretch of road with another set of eyes on the O2 sensors while I ran it down the road). I would think that under such conditions, a fuel problem might surface and cause a lean out, especially with such a long sustained heavy load.

I have no current or pending DTCs, and all OBDII monitors have run and passed.

I have witnessed IAC counts fluctuating all over the map from a low of 23 to a high of (NOT kidding) 47,351. Yet there is no DTC.

I have witnessed intake air temperature readings fluctuate from ambient (80-95*F range) to -38*F and then swing as high as 348*F then back to ambient in under 10 seconds, yet there is no DTC, nor a pending DTC.

I have extensively monitored all other sensors at length and they are reading normal.

I have managed to inspect MOST of the PCM grounds. There are a few I have yet to reach. All grounds I have located are in good order.

Needless to say, this is a bit frustrating. I am sure I'll get it at some point, but this is a very foreign set up for me. Leave it to automotive engineers to take a relatively simple Idea like Port Fuel Injection and make it more complicated than needed for no good reason.

[Idle_Chatter]

I know nothing about this engine, but I did hear on the radio (Sam's Garage) a couple weeks ago that the 4.3L was notorious for problems with a plastic fuel spider on the manifold. Apparently it is a good design, but made of plastic and the spider and poppets are prone to failure with age and heat. Worth a look.

[chevytuff]

Check the intake gasket, They are prone to leaking and cause all kinds of problems. Recently a friend of mine who has an 04 tahoe had set an ses light showed that the motor was running lean according to codes when I hooked my scanner to it The fuel trims were maxed to the positive. The exhaust would burn your eyes. So we knew it was running pig rich. We used some throttle body cleaner around the base of the intake and found it was leaking in four places. 50.00 and 2 hours later no more intake leak. and no more rough running.

[Hog]

I have a good friend with a 2001 Silverado with a 4.3L V6, 153,XXX miles. It's a basic truck; 2wd shortbed, automatic transmission, A/C, and manual everything else. He recently bought it and it has an unusual starting problem. If you turn the key to start it, sometimes it cranks right up and runs like a top. Other times, when you crank it, it acts like it's starved for fuel. It just doesn't start. However, if you turn the key off, then try to start it again, it will crank right over, no problem, and run like a top. I have spent a great deal of time checking this thing over, but I am lost. I have virtually no experience with GM's CFI/CMFI setup, and I feel like there's something I am missing here.

What I have checked:

I have tested the fuel pump relay, and I have even substituted it with a known good relay for comparison. The relay is OK.

I do not have a pressure gauge that will work with this system, so I do not have fuel pressure readings, but I do know that the system will drive the O2 sensors rich at WOT from 0-100+ MPH according to my Genysis (that was on a long stretch of road with another set of eyes on the O2 sensors while I ran it down the road). I would think that under such conditions, a fuel problem might surface and cause a lean out, especially with such a long sustained heavy load.

I have no current or pending DTCs, and all OBDII monitors have run and passed.

I have witnessed IAC counts fluctuating all over the map from a low of 23 to a high of (NOT kidding) 47,351. Yet there is no DTC.

I have witnessed intake air temperature readings fluctuate from ambient (80-95*F range) to -38*F and then swing as high as 348*F then back to ambient in under 10 seconds, yet there is no DTC, nor a pending DTC.

I have extensively monitored all other sensors at length and they are reading normal.

I have managed to inspect MOST of the PCM grounds. There are a few I have yet to reach. All grounds I have located are in good order.

Needless to say, this is a bit frustrating. I am sure I'll get it at some point, but this is a very foreign set up for me. Leave it to automotive engineers to take a relatively simple Idea like Port Fuel Injection and make it more complicated than needed for no good reason.
Fuel pressure should be 58-66 psi. You can drip a bit of gas down the throttlebody if you think you have a bad pump. Remove all teh plastic air intake ducting off teh throttle body, then throw in some gas and crank teh engine in an attempt to start it. I have literally backed a truck into teh garage by dripping gas from a babyfood jar down the t-body witha friend at the wheel. With this truck we had an intermitent start condition with the pump making over 60 psi of pressure. Swapped out the pump and all was good.
I have seen bad pumps produce an intermitent start issue.

Fuel pressure is very important on these CSFI(central port fuel injection).
next time it wont start, immediatley after you release the key you should hear the fuel pump run for 2 seconds. Fuel pressure is used to open the poppets on these trucks, so if the fuel pressure falls under 50 psi, sometimes the poppet wont be forced open.

All injected engines will go rich at WOT becaus ethe PCM enters PE mode, this is why your scan tool shows the mixture to be rich at WOT.

Something may be up with your IAT sensor as well, it seems kind of wonky.

peace
Hog