Josh,

Captain said that negative fuel trim means PCM is trying to lean things out. So if you base your problem off of fuel trim values, it is actually running a bit rich at idle and PCM is trying to compensate for that. Correct, Captain?

However, the spark plug wire seems a likely culprit. If you replaced it did it fix your problem and your fuel trims?

Les

 

Hi Les,

The spark plug wire replacement helped, but I am still getting a lot of small misfires.

Back before OBDII (and with the hotrods we built that had no diagnostics whatever) I'd diagnose other ways; spark plugs, exhaust smell, etc. You write like you might be old enough to remember computerized carburetors They were great fun!

Anyway, were I diagnosing this Blazer using the old methods, I'd say it's a lean burn condition. The exhaust smells lean and the plugs are burning lean.

I was sort of surprised to see the negative fuel trim (in light of what they mean) because that means the computer is not compensating for whatever reason.

The fuel filter needs changed anyway, so I'll do that and see what happens.

Back when I did the tune-up after getting this Blazer in 2009, I found the cap and rotor to be original at around 120,000 miles. The rotor button contact was burnt clean through.

I'd not be surprised at all if this is the original fuel filter. I've seen them go this long (on lower fuel-pressure vehicles, anyway.)

Regards,

Josh

 

Hello,

I'm logging everything here in the hopes that it'll be helpful to someone else sometime.

The air charge sensor/air temp sensor initially looked to improve the problem when unplugged, but further experimentation showed that was wishful thinking.

The only thing that improves driveability at this point is unplugging the MAF.

Currently have the fuel filter's threads soaking in Fluid Film so it'll be easy to change.

Regards,

Josh

 

My MAF sensor was bad, but didn't throw any codes. Same problems, misfire, lack of power etc.


Having 2 of the same engines helps, swapped in a known good one so diagnosing was easy for me.


Changed it and all has been good for over a year now.

 

I've not done any further work on this as I've not had time.

However, I was thinking -- when I first got the truck I put an Accel (or MSD, I forget) coil on it.

Any chance this could be causing a crossfire condition? If I recall the specs, it wasn't much (if any) hotter, but was good to a higher RPM. At the time, I had an idea for a performance build and had the coil so I just put it on.

Regards,

Josh

 

Update:

I installed an old-style distributor clamp. I'd read about the distributor offset and it just seems like the distributor is the root of the problem.

I advanced the offset a degree or two. It's improved dramatically. It's still there, but I think all this is cross-fire.

The distributor gear looks new-ish. There are no vents in the distributor.

I'm thinking this is a replacement aftermarket distributor that was not installed correctly.

I'm also looking at the bushing. I'm simply not sure it's not worn.

MAF: I got to thinking about this and I didn't recall seeing screens. I looked, and sure 'nuff, the screens are gone. The air passing over there thermistors isn't going to be the smoothest for this reason.

I'll start by replacing the distributor. I want that metal one anyway. Will go from there.

Regards,

Josh

 

- Solved -

It's the distributor. Or was. I changed the plugs out. It still missed.

I loosened the new distributor clamp and moved it about one degree, then drove it. It had improved, but still missed a little.

I repeated, and no more misses.

It's slightly more "advanced" than the alignment points indicate it should be. The gear looks just fine, not worn.

The only thing I can figure is that the distributor (or just the gear) must have been replaced at some point with an out-of-spec unit. I'm still going to buy the aluminum one, though.

Anyone know where I can get a 350 throttle plate? I'd like a smaller restrictor but don't want to go hacking on this one. I modify only when I can't buy the piece I want.

How else are these engines detuned?

Regards,

Josh

 

Rotating the distributor accomplishes two things simultaneously: It aligns the camshaft position sensor with the camshaft, AND it aligns the rotor segment with the terminals inside the distributor cap to reduce/eliminate the possibility of crossfire inside the cap. It does NOT affect ignition timing. The adjustment is viewed on a capable scan tool as "CMP retard" or "Camshaft retard". Desired spec is zero degrees, with a tolerance of plus or minus 2 degrees. If it is out of spec, there will be crossfire going on, guaranteed, and it will not set a DTC in memory, unless it's more than ~27 degrees out of spec.


The plate on the back side of the throttle blade is there for a reason. It helps distribute EGR gasses inside the plenum evenly to all cylinders. You're better off leaving it alone.

 

I understand. This is why I put the " 'advanced' " in quotes. The previous AC Delco cap looked like it had been crossfiring.

I found this in a service bulletin. The stated model year didn't include the 2000, but it seemed it would apply there, too and is why I started looking harder at the distributor. I do need to check it on the scan tool as I'm sure I didn't get it right on 0 degrees. Haven't had a chance to do so yet.

Could be you're right. I thought it achieved two purposes, though: Distribute the gasses AND detune the engine a bit. Is this correct?

Mostly I'd like to know why the 2000 4.3 in the Blazer rates 190hp while the ones in the Silverado rate, what is it, 245hp? Mostly I want a bit more torque in the lower end.

Was all the detuning done in the PCM coding on these?

Thank you,

Josh

 

Silverado and Express Van 4.3s were 200HP. Blazers were 190HP. S10's were 180 HP. I understand it, different camshaft and PCM tuning. Think you would be hard-pressed to notice 10HP or 10ftLB difference in a 4000+ pound 4WD vehicle. Better to make what you have run correctly.

245HP is common for the 5.7 Vortec V8.

Distributor alignment marks - these get you on the right distributor tooth, but that is about all (especially if you hold down has been modified or if you have replaced it with a standard one). Your scan tool may not do CMP retard as this is a GM-only PID that is not commonly found on inexpensive scanners or scanner software. Got to remember CMP retard is not part of the base OBD2 parameters that a lot of these scanners are limited to. GM invented and added this PID to OBD2 for their own purposes. Only cheap software I know of that will do it is Car Gauge Pro at $8 (used in conjunction with an Android and bluetooth OBD2 adapter - see BAFX on Amazon). The ever-popular Torque Pro with GM enhanced will not do CMP retard, but it is well worth the $5 if you already have the bluetooth adapter. More pricey softwares like Scan XL Pro, Autoenginuity, HP Tuners, and EFI live, etc. will do it - as will the pricey dedicated scan tools like Snap-on, etc. If you want software that does bidirectional functions like Crank Sensor Relearn and Injector Balance Test, then plan on spending at least several hundred.

If you are not going to actually check and set the CMP retard to zero, you will be much better off replacing the distributor gear to eliminate any wear and using the unmodified factory hold down (or in your case just using the Skip White distributor assembly with the hold down it comes with). Otherwise you really have no idea where it is set. Make sure the distributor gear is not installed 180° out on the shaft, because 13 teeth throws things off. Google for correct orientation of distributor gear on the shaft.