I have a 1993 Jimmy 4WD Vortec 4.3 l with 170,000 mi and a rough idle. Checked all the usual suspects, EGR, IAT, plugs, rotor, cap, wires etc. There are no codes. I checked the fuel pressure and found that it is well within specs initially and during run time. But, if I check the pressure with key on/engine off I get over 55 psi but it drops off IMMEDIATELY when I turn the key off.

Reading a lot of the posts it seemed to be down to the fpr, nut kit or maybe the fuel pump check valve. I pulled the IMTV to check the plenum and saw a slight bit of washing on the passenger side, so maybe the fpr is leaking. Picked up the parts and pulled the plenum today. Here is what I found (pics are attached, I hope):

• Upper plenum uniformly dirty
• Lower intake has slight washing where I saw it through the IMTV on the passenger side but nowhere else
• The driver's side has a slight (spotty) uniform washing effect
• There is a little fuel pooling in both sides
• The entire lower intake looks "wet" although I don't know how it is suppost to look.

I turned on the key to prime the system. The pump normally runs about 3-5 seconds if the engine has been sitting overnight, then stops. Always starts right up. If the engine has been down for a short time I always prime it for 1-2 seconds and it starts right up. While running the pump I don't see any fuel leaking anywhere. It runs for the few seconds, shuts down and stays down. When I turn the key off the pressure drops but no leakage noticed anywhere at anytime in the process.

Since the washing seems so slight I am wondering if it is just the beginning of a problem or maybe just normal. Should I go ahead with the expense and replace the fpr and now maybe the fuels lines or leave it alone and troubleshoot elsewhere? And where would that be?

 

Since you have it open I would replace them both anyway the FPR and the Nut Kit. If your losing pressure, it may not be where you see it. It could be leaking from the return line on the nut kit otherwise look for leaks under the truck, if nothing then I guarentee its the Pulsator in the fuel tank. The pulsator is a piece on rubber that is between the fuel pump and the fuel line. They tend to wear out, and the ethanol in the gas makes that process go a little faster. Check there if you dont notice any leaks in the plenum and under the truck. The bad part is that you have to drop the tank to find out.

 

Thanks for the feedback. I haven't been able to work on the truck for the last two days. Went out tonight to verify the fuel pressure again. I have not done it since I took apart manifold. Pressure jumps up to 55-60 lbs key on/no start and drop off immediately when the fuel pump turns off after a couple of seconds. My recollection is that on prior testing the pressure stayed up as long as the key was on and only dropped off when I turned the key off. Don't know if the dismantling had anything to do with the changes but there they are.

Interesting thing though is that on the earlier test and whenever I started the truck the fuel pump would run for a good 5 to 10 seconds, then shut off while maintaining pressure until the key was turned off. Now, as I said, it runs for about 2 secs and shuts off with a drop in pressure. Strange. Your idea of the "Pulsator" may be the answer because I don't see any leaks. Although I don't know if maybe the fuel is going back down the return line?

 

Well my thought on the washing on the drivers side would be gas leaking from the return line on the nut kit. The loss in pressure in my opinion is because of the pulsator located in the fuel tank

 

Before you move to the tank, you should look more towards the intake right now. The fuel washing is definetly a sign of a problem. Because of the passenger side, I would probably replace the fuel pressure regulator.

As far as the drivers side, look closely at the underside of the pressure and return lines, and also the lines for the poppets. My return line had a tiny tiny hole in it in the bottom from rubbing on the poppet line. It would spray out intermittently. The pulsator could be a problem also, but I would be concerned with the fuel washing on both sides of the intake right now.

 

Thanks for the info. I did replace the FPR even though the washing wasn't very heavy on the passenger side. I did see some rubbing between the spider and the fuel infeed line but could not tell if it was worn through. I pulled the electrical connector off the CPI and then the rubber "bushing" and found a little fuel in that area. I dried it and repressurized the system, cranked the engine a bit. When I took the connector off again. I had a little more fuel in that area. Leads me to believe that I have a leaking CPI.

Don't have $300 to replace the CPI/Nut Kit so I put it back together. Have found some remanufactured CPIs on line with the same warranty as the new one from a reputable manufacturer. I may give one of those a try.

As far as the pulsator, all my testing indicates that it may be bad but since the only issued is loss of pressure when the fuel pump is off I will probably let "sleeping dogs lie" until I have another reason to drop the fuel tank.