Not to lead you to the answer, but the info I have might be pointing me there or to a cam sensor (slim chance tho on the cps).

Anyways, truck has random misfire when cold under lite throttle. After 20 min or so, miss goes away. I have owned the truck for 2K miles and hadmiss when I got it, but not bad. Miss has gotten progressively worse with the same conditions as stated above. Finally set a p0300 code last nite when cruise was on and truck was going up a slight hill with the converter locked up and engine misfiring.

SO, after much research here and on automotiveforums, I borrowed my buddys Actron fuel press tester and got the following results:

Warm engine - 40min after a 30min drive

key on and engine off- jumps to 64 psi with pump on, as soon as pump turns off drops to 58psi, let sit for 1 min and psi creeps up to 64psi and stops.

engine running - 56psi, shut engine off- drops to 53psi and creeps back to 56psi after 1min

COLD engine - let sit for 4hrs

key on engine off- jumps to 64 psi with pump on, as soon as pump turns off drops to 61psi, let sit for 1.5 min and psidrops up to 46psi and levels off.

engine running - 56psi, shut engine off- drops to 54psi and creeps back to 60psi after 1min

Wanted to get psi reading while driving, but gauge wouldnt reach outside of the hood for me to drive truck and watch it!


Anyways, as for parts replaced, I have changed fuel pump assy. (with AZ brand tho), plugs wirescap rotor (with prem AZ brand,bosch? i think)about 1.5 years ago when my buddy owned it.

other thread about cam pos sensor I read stated that in open loop engine relies on cps (if it is bad, eng runs bad). Once in closed loop, engine will not use cps and will run better. If this is true it kinda fits for my prob, but I would think my PCM would go into closed loop after the thermostat cycles once. My truck misfires for 10 or more thermostat cycles before the miss goes away.

all right, let me know what you think and if you need more info I will do my best to get it tonite. I will be fixing this thing in the next few days, but I dont just want to throw parts at it. Thanks for your opinions! - Mike

 

Bueler?...Bueler?...Bueler?...Frye?...Frye?...Frye ?...

 

Must have missed this earlier today. Anyway, that is kind of odd how your fuel pressure jumps around like it does, but it doesn't seem like its all that bad. How does the pressure hold after say a half hour after shutting the engine down, does it remain at 46psi? The drop of 18psi in 90 seconds doesn't sound too good. To rule out bleed down through the pump, you can disconnect the threaded connection at the fuel pump and isolate the pump from the rest of the fuel system. If you still see the pressure drop, then it is a problem in the fuel pump assembly in the tank.

I am kind of wondering if you don't have a cold injector spray problem. Have you tried some fuel injector cleaner through the tank? If so, did it make any difference? You could try a professional fuel injector cleaning, but at $90 (going rate around me). My replacement spider for my '00 Bravada cost me around $200 (it's been awhile). It's up to you what you want to try first. In my opinion, a professional cleaning will buy you some time and money, but won't be a permanent fix. If you stick to a bottle of over the counter injector cleaner every oil change, it may last quite a while.

HERE is what I went through to diagnose my injector troubles (take note of post #9 in that thread). I was already pulling the intake apart to replace the lower intake manifold gaskets so I ran a few tests on the injectors.

 

Right, I think this is a great reason to finally make my DIY FI cleaning kit that I have always wanted! If I get it to work, I'll post a DIY how to. It would consist of an old R-12 can dispenser (that punctures the side) and convert the end to a schrader valve for the fuel rail. Hook up can of NAPA fuel system cleaner, De-energize the FP, pinch the return line and fire the engine up until the cleaner is gone. Might work good...?

 

You would need to maintain ~55psi in the container. Any additional pressure will discharge back to the tank. You can do this quite easily with a pressurized air supply and a regulator. The professional kits have an expansion tank of sorts that gets used to maintain a set pressure. And then there are others that require disconnecting and plugging the return line to the tank.

 

Right, I think it's possible to pinch a rubber section of the return line on my truck IIRC. 3M version of aerosol cleaning agent states 100psi, then I guess you regulate it down with your vehicle interface device. Probably wont use 100% of the cleaner, but if I can do this project on the cheap it might still be worth it.

 

bought all parts to do DIY FI cleaning kit with pressurized can of solvent. It will consist of a 134a refrigerant can dispenser (side puncture clamp type) to an old 134a hose dispenser. I chopped the car side of the 134a hose off and pushed in a brass barb. This brass barb goes to a brass ball valve and then to a brass threaded "tee" that I installed beneath the gauge of my Actron FP tester. If all goes as planned I should be able to install a can of "Pro Clean" injector cleaner on the 134a dispenser and add it the the fuel rail via the above mentioned assy.

Pull FP fuse, clamp off return line, add pressure to the fuel rail via the brass ball valve (try to keep 50-60psi? or enough to get it to start), run engine till it quits. I'll try to keep the can warm to increase pressure. I'll post back to let you know how it goes.