Hey guys, trying to pinpoint the issue on my father's '98 1500. It's had a slight misfire at idle since my father had the guys in his shop swap a new engine in last year. It was originally a 5.2 however they sourced a 5.9 by accident and ended up installing it with the 5.9 (with the proper flex plate, I made sure of that).

It's ran well over the last year except for the slight misfire at idle. Yesterday it started missing more severely and it was noticeable while driving after a heavy rain, so I attributed it to moisture in the distributor cap. I did a full tune up with brass cap and rotor set, new wires, NGK coppers, and followed the TSB for rerouting the plug wires... These Cubans really had no freaking clue when they did the engine, they used Autolite platinum's that were gapped at .065" and had the plug wires ran like complete garbage.

Now it's running better but it still has a slight miss at idle and is still missing under a load. With my scanner the idle fluctuates between 650 and 608 RPM's and it's still showing a cylinder 1 and cylinder 2 misfire, plus the P0300 multiple cylinder misfire code. and is showing a RICH condition (the old plugs back this prognosis up).

It is still running the 5.2 PCM, however it's ran great for the last year with the exception of a slight miss at idle. For the slight idle miss it doesn't throw a check engine light.

I'm moving onto injectors next, however I was wondering if these idiots that did the engine swap messed with the distributor... If they did, obviously the fuel sync would be off. Would this cause the slight miss at idle it's had since the engine swap? He's also complained about terrible gas mileage since the new engine was put in (9-10 vs his usual 11-13, lol).

Any suggestions?

 

Can your scanner read fuel sync? Check that before spending any money. Also, make sure fuel pressure is where it should be. (49psi @ idle)

Since you think it is running rich..... how old is the O2 sensor? Has it been replaced in recent memory? (I don't think there is a difference in them between the two engine sizes, but, crazier things have happened.....)

Does it run good when it's cold, and then develop the miss as it warms up? Or, is it always there? (I could see it having a miss when cold, as it is running off of tables, and not paying any attention to the O2 sensor.... so, as the PCM thinks its still a 5.2, it would run a bit lean at idle, and could give you a miss..... also possible that when warm, it STILL ignores the O2 at idle..... not sure on that one though.)

 

Check this se if it helps http://www.obd-codes.com/p0300.

Dave

 

the 5.2 pcm can run the 5.9 engine on anything except cold startup and WOT. The O2 sensor will tell the pcm how many milliseconds to fire the injectors, but at cold startup and WOT, it is ignored and the pcm uses tabled values, which are from a smaller displacement engine. Injectors are the same for both engines, btw.

SCT tuner can rewrite a 5.2 pcm to run a 5.9 engine.
Just curious as you didn't mention it, but the harmonic damper is also weighted on the 5.9, but not on the 5.2. I'm sure you got it if they swapped the whole engine in, but do double check.

cylinders 1&2 are also at the end of the fuel rails, and with a non return based fuel system, the bulk of the crud could easily be pushed down to the ends and collect gunk the the injector baskets.

might want to check the torque on the intake manifold bolts too, small leaks can lean a particular cylinder out and cause issues. 12 ft/lbs. As the lean cylinder fires, the O2 sensor tells the pcm to use more fuel, then the next is too rich, and so it pulls fuel, and around it goes. Actually, I'd be checking for vac leaks, as those will cause you to run rich/lean/rich/lean in a crazy cycle that could also effect idle rpm fluctuations.
Po
Verify the TPS signal isn't "noisy" at idle with your scanner. No more than a slight .01 or .02v blip (depending on scanner accuracy) should be tolerated.

Cylinder head cracks between the seats can also case hard to find misfires, but those typically manifest in 7&8 first, as they typically run hotter as the cat gets older and clogs up.

Bent valves can cause misfires, or bad seats, also too much carbon from bad valve seals leaking oil.

Weak coil, if you are getting the P300 random misfire code. Plus poor fuel pressure.

That's about all I can think of offhand, Junior.

 

Well It's continuing to run terrible, then poor, then terrible, then poor again. If you punch it she hauls azz, but at idle, cruising, and normal driving the miss is noticeable.

I cleared the codes then took it for a drive. While driving it threw a "system too lean" code (I think it was P0171, I don't remember though). I couldn't get it to throw any misfire codes but I only drove it for about 10 minutes (although I drove it hard).

It literally sounds like it's got a nice cam in it at idle when it's missing the worst. I have noticed it seems to get worse as it warms up, I'm not sure if this after the system enters closed loop or not. You can smell unburnt fuel and it smokes some as well.

My scanner says "Current sync: Okay" I'm guessing this is fuel sync, there's no other sync that I'm aware of.

TPS at idle is .84v and increases steadily with no fluctuations.

Right now, I really want to say the upstream oxygen sensor is bad. My dad thinks it's the injectors but I disagree, the fact that it will run well then run poor then run well again and that it's detecting a rich condition then a lean condition really points to more of an electrical input issue to me more so than an injector failure.

The cam sensor is also on my mind however in my experience the cam sensor will usually throw a code.

 

I think you are on the right track with the O2 sensor. Try that first.

 

I had the same problem on my 1996. And this checks out with your father's dilemma.

My reason:
The injector connectors on the wiring harness. i swapped injectors and when i took the connectors off, there are these SMALL WIRE CLIPS on the bottom of the plastic injector connectors (EV1 connectors). Some of them flew off when i was taking the connectors off and I never replaced them. WELL they help keep the injectors on there nice and SNUG. If they are not on there nice and snug, then you will get a misfire every time. For new connectors with the clips and PROBLEM SOLVED.


ABOUT THE TRUCK SOUNDING LIKE IT HAS A CAM:
1. When I had to replace the complete wiring harness on my 1996 5.2L, I put the relays in from a junkyard PDC. LITTLE DID I KNOW, every year and even within years has different PDC relay/fuse diagrams. I had the wrong relay in the wrong spot and the idle sounded pretty mean.

you don't believe that, take a look at the underside of the PDC cover of your truck and a friend's secondgen. Guaranteed they are different.

I would bet this is a relay problem and possibly the injector connectors since the engine was swapped. They probably ripped the injector connectors off, and trying to find those little wire clips is damn near impossible once they fall off.