2003 3.8 with about 175,000 miles. Code PO300, Long post, I need further advice. Last week on the way to work, suddenly lost power, running very rough, idle very rough, engine shaking. First thought, bad coil pack. I replaced ALL 3 coil packs and ALL spark plug wires back in March with ZZP brand, also new plugs at the same time. So, I put my old coil packs back on. Still SAME ISSUE, rough, rough running and idle. Then I replaced spark plugs, still the same issue. Checked the injectors, they are original, they looked dirty, they ALL worked but I don’t know how well. I replaced them with a Re-Man injectors, still same issue, idle rough, engine shaking. Checked fuel pressure, it was dropping off with key on and key off. I replaced the Fuel Pressure Regulator, brand new one, still slowly leaking off pressure. Took it back, got another brand from another parts store, still leaking off fuel pressure, took it back, put old one back on. Still leaking off fuel pressure. I replaced the Fuel Pump with a brand new one. Still same issues, leaking off fuel pressure, idle rough, very low power, leaking off fuel pressure. SHORT TERM and LONG TERM fuel trims are BOTH DOUBLE DIGIT in the POSITIVE, short like around 20 and long about 14. Just to eliminate stuff, I put on a used Idle Air Control and MAF Sensor, tested the catalytic convertor, CAT IS GOOD. I also Smoke Tested for vacuum leaks. Found 3 VERY small ones, One In Rear Exhaust Manifold Pipe, One In The Front Manifold Pipe, and One At The Throttle Body, where I got a fitting screwed in, in the bottom, body of the throttle body, and thats a long story. Anyway, I serious doubt those 3 VERY small leaks will or would cause my issues, certainly would not think, they are responsible for the Fuel Pressure Drop. I finally capped off the return line, just after the regulator, Keyed on, and fuel pressure shot up to 100 psi. And thats where am at now, Fuel Pressure has now been dropping slowly about 2.3 Lbs PSI every 10 minutes. Can It Be Something Internally Wrong??? I know my old injectors, were part of the problem, along with a weak fuel pump, along with a weak pressure regulator. So could a combo of all 3 have caused an Internal Engine Problem ?? What internally could cause a slow fuel pressure drop ? Anything else that could cause a slow fuel pressure drop? I have my monte for over 7 years now and have had it LONGER THAN ANY OTHER VEHICLE !! Also, I got another, NEW fuel pressure regulator on it, makes the 4th one.

 

To sum it up to make sure I got it right, the car has ok fuel pressure when you first start it but as it runs, the pressure slowly drops and creates high fuel trims as it falls below spec pressure?

Is it still hitting 100 psi when you first turn the key on? It shouldn't ever be spiking that high. What is the fuel pressure you're seeing while it's running ok and once its run long enough to start idling rough?

 

The fuel pressure when it runs is between 50 and maybe 58, but the needle on my gauge constantly flutters when it’s running. The pressure only drops when turned off, then it drops about 2.3 lbs psi per 10 minutes. I capped off the return fuel line just to try and isolate the fuel pressure drop problem. So when I capped it off as a test, thats when it went to 100 psi, this was only one time, just a test. Also, replaced my fuel filter at the same time as the pump.
I can also hear a noticeable ticking sound from the engine, but can’t tell from where, sounds internal
though. It idles really bad from cold start, engine shaking from cold start as well. Both fuel trim numbers being high point to lack of fuel. But where is the fuel/pressure going ??

 

Change the fuel filter? I bought reman injectors for my 94 jeep and one was bad, and the engine sounded unhappy. I had Injector Repair LLC flush the injectors, for ~$15 or so per injector and it fixed my issues with the jeep. I don't trust reman injectors now without a flow test. My father has a 6th gen LS, he had issues with his car running rough, losing power, and dying. His fix for awhile was to open the gas cap and then put it back on. Eventually bought a new gas cap, not sure if the issue was back pressure, though eventually he had his injectors flushed and it fixed his problems.

I'd check your fuel lines for any small leaks, but my assumption is a small fuel pressure drop is normal over time, it's the rate of pressure drop which is important. But my guess is an injector issue

 

I appreciate the replies thus far. Chances of my injectors and the reman ones both sets being bad is probably very slim. The reman ones I purchased, are from a trusted seller on Ebay, matter a fact, I already purchased a set of them from the same seller, for my 2004 Durango that had a hard start issue. His injectors fixed that problem. I did replace the fuel filter at the same time as the pump. I’ve check the fuel lines for leaks, of course can’t find any and can’t smell any gas leaks what so ever.

 

Assuming the 50ish reading is engine on and 58 is engine off (but key on), that sounds good IMO. How much gauge movement are you talking and is it a fluid damped gauge? If it's just a needle in air, some light flutter isnt unusual, especially if the fluctuations are small. Of course if its swinging all over the place then something else is wrong.

Im not sure what the GM spec is for bleed down, but that doesnt sound bad TBH. If something were really broken (injector stuck open, etc) it would be bleeding down far quicker.

Since it sounds like the issue is while running (and isnt just hard restarts), I personally wouldn't very chasing this slow of a key off leak.

Oh wow, missed the part that you capped it off. IMO I'd be very worried the regulator could get damaged with that kind of pressure inside as that's way over it's normal operating pressure.

Is it always running like this or is it just after the cold start?

Another thing to think of since it sounds like you've ruled out fuel delivery issues - if a single (or couple) cylinders was completely misfiring, it would be passing a lot of raw oxygen into the exhaust which will be picked up by the O2 sensor as running lean (as theres only 1 O2 sensor for all 6 cylinders) causing it to over fuel and thus make the other cylinders run worse as well.

Have you tried an IR temp gun on the exhaust primaries to see if one or more are far colder than the others? I know you've already replaced a lot of the fuel and ignition related components, but it could be something totally off the wall like a dead circuit in the icm or a driver in the pcm, bad wire connection, etc. Just a thought of something else to check.

 

I tested everything possible, I even ruled out the my gauge by testing it on my Durango, pressure holds great, like fuel pressure should. I just now did another check, I removed the fuel rail from the engine and left the injectors installed on the rail, in plain easy view, keyed on to pressurize and NO leaks at the injectors or rail, but pressure was still leaking off, again maybe about 2.5 psi every ten minutes. So, it has to be leaking off, after the rail, between the rail and fuel pump, but as of now I still can’t find it. Bad check valve in new pump ?? Maybe. It’s going to be 3 days before I can check things again. Thanks everybody for help !

 

Does anybody know what size the plastic fuel line is ??

 

Looks like the supply line is 3/8” and the other 2, return and vapor is 5/16” . I guess thats the sizes of both metal and plastic lines ??

 

Just curious, why the concern about the bleed down after shutting it off? Have you been able to find the GM spec for acceptable rate for this test on this vehicle? The amount of bleed off you mentioned seems pretty slow and reasonable. Even if the check valve in the pump is allowing a little leakage back through, that isnt a concern for the engine running, just means it'll take a little longer to prime / crank at next start.