1999 Tahoe 5.7 misfire and fuel pressure question

Wow! I am thrilled to finally get on this forum!! I bought a 99 2-door Tahoe with 5.7 Vortec a few months ago. immediately, I noticed lower intake was leaking internally. So I dug in, and replaced the lower intake gaskets, and decided to replace the crusty nasty old spider injector unit with the newer MPFI conversion unit. All back together. No leaks. Truck has always had a slight misfire since I bought, and continued after the upgrade. Fuel pressure would hit 60 psi with key on, then drop immediately to zero. And running pressure was about 48 psi. Other than misfire, truck ran great. I then swapped the fuel pump assembly 2 weeks ago with a new ACdelco Unit. Now fuel pressure hits 62 psi with key on and holds there for a long time. But idle pressure is now 51-52 psi. That's it. I read everywhere that FP needs to be 55-60 for proper running. New Spider with FPR, New fuel pump and fuel filter. New plugs, wires, cap and rotor. New coil and ignition module and hall & effect switch. How else to get pressure up? Is 51-52 psi okay on the MPFI units?? Would this cause the slight misfire at idle? The misfire does NOT set off any codes or CEL. You can feel the slight misfire at idle and only when warmed up. With the scan tool, it is cylinder 3 that gets all the misfire counts. 2 Intertwined questions here I know, but not sure where to turn. Truck runs like a champ! lots of power, but slight misfire at idle only. Thanks in advance!!
Jason

To hopefully get some direction with the misfire...what do your fuel trims look like? If they get better when you increase RPM, could be a vacuum leak. Which may be the reason why it runs great off idle.
HTH

*shaka*

Hey sir, welcome to the forums!

I think FlyinHawaiian is on the right track with fuel trims. Knowing how they behave at idle (during the miss) and at 2500 RPM may answer a lot of questions as far as the misfire AND the fuel pressure.

I don't know the spec for these converted systems off the top of my head, but 51-52 PSI sounds OK. Unless the fuel trims are significantly positive on both banks, then I probably wouldn't associate the fuel pressure with the misfire.

The misfire at hot idle can point to a vacuum leak, or possibly some kind of mechanical problem. This would not be the first 5.7L I've seen with carbon buildup and/or valve sealing issues. You could put a vacuum gauge on it while idling hot and see if the gauge needle twitches. The overall vacuum measurement would be good to know, too.

Thank you for the pointers. I did check fuel trim a bit back. I will check again at the different idle speeds either this afternoon or tomorrow. If I recall, the short tern fuel trim for both banks went positive/negative evenly. +1 then -1, +2 then -2 etc. I remember LT trim was a bit different. If I recalled, I think left bank would up to 7 or 8 percent. Can't remember is it was neg or pos. I will check again and repost. Thanks!!

I'm with Tyler being old school a single cylinder misfire at idle that smoothes out always makes me think valves or a vacum leak close to the inlet valve. An exhaust valve that is leaking slightly will give an idle misfire but get smoother as the RPM increases.
If the vacuum gauge shows nothing out of line and a water spray on the manifold to head joint comes up empty then look to do a compression or leak down test if you have reasonable access to the spark plug.

Keep in mind you can never 100% depend on misfire codes being always pointing to the correct cylinder sometimes they point to the cylinder next in the firing order.

Thanks again guys for responding. I very much appreciate it. So I warmed up the Tahoe and hooked up my scanmaster. At idle (and I could feel the shake, STFT Bank 1 and STFT Bank 2 would both switch between -1 to 0 then -2 to 0, +1 then 0, -1 then zero etc. LTFT Bank 1 basically stayed at -5 and LTFT Bank 2 stayed at -10. then I brought it up to 2500 rpm as suggested. motor feels smooth, can't "detect" any misfires. So at 2500 rpm, STFT Bank 1 and STFT Bank 2 went between 1.0 then 0. Then -2 to 0 etc. same sequence. LTFT Bank 1 was -5 and Bank 2 was -5.

Intake Manifold Ab pressure was 9.2 at idle. Not sure if that helps. I think cylinder number 3 is Bank 1. I pulled the plugs. They are all about a month old. They all looked very good. Clean. Cylinder 3 plug had a weird orange color on half of the porcalin on the inside. I know that the FPR and the individual injectors are not leaking. Maybe you guys are onto something with valve seals. Truck has 122k miles. I hope the info stated will make some sense to you gurus. I can live with the bit of misfire, but it drives me nuts at a light or something. Maybe be contributing to poorer gas mileage. Thanks again for all your time
J

Thanks for getting back to us with the trims! I think we can pretty safely rule out vacuum leaks and a fuel pressure problem, as neither would cause your trims to run negative.

Let us know how the vacuum gauge test goes, if you get the opportunity. We can interpret the overall vacuum level from the MAP sensor (which, at 9.2 at idle, is pretty darn good), but the scan data won't show the twitches of a needle on a manual gauge, if there are any.

Thinking back to other 5.7s I've seen, did you get the chance to check the CMP Retard data PID after installing the new intake? Usually, a problem here will result in problems under load, but can manifest in other ways. Ideally, we'd like to see CMP Retard at 0 +/- 2 degrees.

Hmm....and I thought the fuel pressure should be higher. When trims go negative, means pulling less fuel, right? Computer attempts to lean it out? (trying to remember, from all of Paul's YouTube videos) lol. I will hook up vac guage. Was raining hard today. Dumb question, as most of my work is done on a 71 Buick 455. Simple engine. Where do I connect vac guage on the 5.7 vortec? Not your standard carb ports. When I pulled the intake for the lower gaskets, I basically scribed 2 separate marks to put the distributor back in exact! and I did. However, I don't know what the timing was prior. I reinstalled it perfectly. I did not get any codes. I assume it's good, but my scantool only tells me timing advance, not the actual initial timing. I will get on vac, once given a pointer where to connect. Atleast, from your comments, the possible lower fuel pressure is NOT causing the misfire. Thanks! Love this forum.

jaystoy wrote: Hmm....and I thought the fuel pressure should be higher. When trims go negative, means pulling less fuel, right? Computer attempts to lean it out? (trying to remember, from all of Paul's YouTube videos) lol.

Yep! The computer is injecting less fuel into the engine, in response to a measured rich condition. I can check Mitchell on Monday, but the Snap-On Troubleshooter says KOEO fuel pressure should be 54-60 PSI, and should drop to 53-56 at idle. The numbers you saw are close enough for me My reasoning here is that, if the fuel pressure is low, then the PCM wouldn't need to take fuel away from the engine.

Where do I connect vac guage on the 5.7 vortec?

Not a dumb question I'd go for the brake booster hose first, with the check valve at the booster end of the hose removed. You could also go for the purge valve port, but that'd take more work.

When I pulled the intake for the lower gaskets, I basically scribed 2 separate marks to put the distributor back in exact! and I did. However, I don't know what the timing was prior. I reinstalled it perfectly. I did not get any codes. I assume it's good, but my scantool only tells me timing advance, not the actual initial timing.

No problem, I think you did it exactly right! Sorry, I thought you were looking at some Enhanced data, so checking the CMP Retard would be quick and easy. My mistake :blush: We might come back to that later, but I say don't worry about it for now. The code you'd see if it were way would would be the infamous P1345 Cam/Crank Correlation.

Let us know what you find with the vacuum gauge, and we can go from there!

Only time I have seen orange colour on a single spark plug was a cylinder head gasket leak -- it looked like a weird stain rather than a deposit. That engine had slight misfire on initial start up and was filled with red-orange OAT coolant.