4.3L Fuel Pressure/Hard Starting Question

ricobeach · December 18, 2006

Hi everyone,

Hopefully you gurus can give me some pointers on what to try next with my 2000 Silverado 4.3L (105,000 mi) hard starting problem.

For the past month, about 60% of the time it has been harder to start then usual (longer time cranking before starting). It doesn't seem to matter much if the engine is cold or warm. Sometimes it starts immediately, sometimes it needs to crank for 3-5 seconds (it has never failed to start). Once started and warmed up it runs reasonably well but occasionally runs a bit rough for a few seconds (noticable at stop lights but tachometer does not jump at all).

I have replaced the normal items such as spark plugs, wires, and distributor cap and rotor. They were not worn too badly but need swapping due to mileage and to take them out of the equation. Fuel filter replaced at the 93k mark and battery only 13 months old.

It also had a "Service Engine Soon" light come on about 15 miles after a refueling. It went away shorly after I reset the gas cap. However a scan tool indicated a "P0300" code stored (multi cylinder misfire detected). This occured about 5 weeks ago and has not occured again.

I did test the fuel pressure this weekend and this showed a problem. During the initial priming of the pump (key turned to "on" position, not started yet), the pressure read 60 psi. Once the whine of the pump stopped, the pressure dropped to zero. I obviously have a leak somewhere. Once the engine was started, the pressure read 51 psi while idling. When gas pedal was pushed, the pressure would increase to about 55 psi but quickly settled back down to 51 psi. I assume that this indicates that the fuel pressure regulator is working. Any ideas on other things to check? Is it possible to check whether the fuel pump check value has failed and simply lets fuel cycle back down through the return line, thus depleting the fuel pressure to zero before startup?

The fuel pump was replaced 10 months ago with a aftermarket pump by a professional shop. If there is an easy way to check whether the pump is having problems, please let me know. The pump is still under warranty so I could have the shop remove it and replace it (hopefully).

Sorry for the long description but I wanted to get the symptoms across with as many facts as possible. Thanks for any suggestions for me to try next.

Rico

GM-Tech · December 18, 2006

Once the whine of the pump stopped, the pressure dropped to zero.

This sounds like a bad check valve in the fuel pump (new or not), and the cause of your starting problem, but I can only be 90% sure without added diagosis. If it wasn't a 4.3, I might feel more positive but these are know for fuse pressure regulator leaks, plus two plastic lines in the plenum that feed to the injector assembly.

slowpokebanshee · December 19, 2006

the proper way to diagnose "loss of rest pressure" concern is to restrict the fuel feed pipe, and return pipe and see if you still lose pressure, if it holds pressure until you open the ckt then the pump is to blame, but I would suspect the regulator due to the drivability symptoms you described, if you have an inspection mirror and a flashlight, you can either remove the throttle body, or intake hose assy and hold wide open throttle, but if the regulator is leaking that bad you can actually see it drip, and/or the intake manifold under the regulator will be "washed" from fuel and clean, your misfire concern may or may not be related, w/out viewing your freeze frame data, hard to tell, but you may or may not be a candidate for "special policy" injectors retrofit, I recommend take it to the dealer and check for special policy candidate, because if they replace your injectors (free) it comes w/new regulator........if you need more info feel free to e-mail me at:

[email protected]

ricobeach · December 19, 2006

Thanks GM-Tech and slowpokebanshee for the replys.

I will try to further narrow the problem down by attempting to check the fuel pressure regulator this weekend per Slowpokebanshee's comments. If I cannot due to Christmas gatherings, I will have to take it in for a proper diagnosis and repair. I'm hoping that it is the fuel pump (for warranty replacement) but I also agree with GM-Tech that the 4.3l has had problems with the fuel pressure regulator leaking. This was mentioned a lot in my google search findings.

Whenever I get this resolved, I will post the results.

Happy Holidays everyone,

Rico

ricobeach · January 3, 2007

Quick update to my 4.3l hard starting and fuel pressure problem. It turns out that the CPI (central port injection) system was bad. The poppets and lines were leaking so I had to have the entire CPI system replaced. So far it seems to have fixed the problems. Unfortunately I did have to pay out of pocket for the repair. Hopefully nothing else goes wrong with this vehicle for quite awhile.

Rico