Truck excellent condition; suddenly runs terrible. Well maintained, new parts.
 

I have an '01 Jimmy with under 95,000 miles.

I get a random misfire code, a rough idle, and it usually backfires within about 60 seconds, usually starts popping first but no knocking. A couple days prior, I had started it with a rag in the end of a loose intake tube (accidental), but it ran great for the 5-10 minutes it was running, even when revved up and also while idling. Then I installed new sway bar bushing kit, finished installing K&N intake, and flashed the ECU (which I have flashed back to stock to diagnose the current problem); It ran as mentioned when I next started it.

Although I later found a rag in the engine compartment below the tube opening (as if it fell out), I am suspecting there may have been another rag (due to the unexplainable loss of performance). I looked into the upper intake plenum with a light and mirror, but couldn't see a rag, though it was hard to see well. I was told the red shop rag likely couldn't have gotten far if it was sucked up, though I have read horror stories with other (bigger) engines. Likewise, if I sucked up a rag, it wouldn't explain how everything ran so well during the run when the rag would have been sucked up. I am getting a camera probe later this week to get a better look.

I have new MSD 8.5mm wires and a used Accel cap and rotor from my last Blazer. The cap and rotor have less than 5,000 miles on them, but I cleaned them very well anyway, with no noticeable beneficial effect. The cap had no apparent defect, e.g., a hairline crack, but the rotor's brass piece looked dull (I sanded the whole tip area down to clean shiny metal). Unplugging the MAF sensor seems to help a bit, and it was not on at all during the run when I may have sucked up the rag. (The MAF sensor resistors and housing are cleaned like new, though I haven't tested its resistance.) I also am using new Bosch Ir Fusion plugs, an Accel coil, and an MSD 6-AL ignition (which I successfully bypassed to test, but to no avail). I also installed a new Delco fuel pump and filter a couple months ago and the fuel system tested fine, but I will check again for the hell of it. I have not examined a plug yet, but I suppose I should to get an indication of what is going on.

I'm thinking the timing is causing the detonation, or maybe intermittent spark that is ineffective, causing flooding which eventually ignites. Although I had the cap and rotor off, that shouldn't disturb timing. I flashed the ECU, but the timing mods were very minor to begin with, and I flashed it back to stock anyway.

One other thing: I sprayed everything in the engine compartment with WD-40 to prevent corrosion of plastic/metal/rubber. I was thinking I may have shorted a sensor with the spray, though this has never been a problem in the past. But what sensor?

The fact that there is no specific cylinder for the misfire leads me to believe there is a simple malfunction with a primary spark/fuel apparatus, but I can't seem to pinpoint it. I made no major changes to those systems and yet suddenly it started running terrible. I was thinking of testing with a new ECU, though I don't suspect that will solve anything. It runs better at higher RPM, but I'm guessing that is naturally the case. Any tips would be appreciated.

I have a marine intake in the mail, which I ordered to give me an excuse to tear off the stock intake manifold to look for a rag.

Thanks!

https://blazerforum.com/forum/images...icon_moped.gif

What are you using to "flash" the ECM? And what are you changing? Is P0300 the only DTC you're getting?

There are three things to keep in mind:
1) This engine doesn't play nice with aftermarket parts.
2) Just because a part is new, or it looks good, is no guarantee that it is good.
3) See #1

P0300 is almost always caused by a fault in secondary ignition. These are the secondary ignition parts that must be correct before continuing with diagnostics:
Spark plugs: AC Delco 41-993 with a gap of .060"
Distributor cap & rotor: Needs to be AC Delco, (yes, it does make a difference). It needs a dab of silicone dielectric grease on each terminal, inside and out.
Plug wires: AC Delco, Autolite professional series, Belden, (good quality parts, no AZ stuff).
Ignition coil: The OEM style coil is capable of producing 40K+ volts, (way more than necessary).

P0300 can also be caused by incorrect camshaft retard, worn distributor shaft bushings, and low fuel pressure.

Camshaft retard streaming data is viewed on a capable scan tool, (the el cheapo $500 jobbies can't do it). Preferred spec is zero degrees with a tolerance of +or- 2 degrees. It is adjusted by rotating the distributor, and it's part of the job when installing the distributor to check and adjust it. If it's out of spec, crossfire will occur inside the cap causing the engine to misfire. The farther off it is, the more crossfire. The only DTC that might be set, is P0300. If camshaft retard is off ~27 degrees or more in either direction, P1345 will set in memory and turn the SES light on.

Distributor shaft bushings: There must be no radial play. Best to check it with an oscilloscope.

Fuel pressure: Key ON, engine OFF, (fuel pump will run for ~2 seconds and shut off). Fuel pressure must be checked when the pump is running: Pressure must be 60psi to 66psi. It must remain above 55psi for at least 10 minutes after the pump shuts off.

FYI: If you remove the lower intake, use a Felpro MS98002T gasket set, follow the instructions, tightening sequence, and torque specs to a "T"
Attachment 31809


Post your results.

I use Jet DST to tune the ECU.

Thanks for all the info. I will dive in as soon as I finish this last tie tod on my mom's minivan.

You say use dielectric grease on the inside terminals of the cap???

And yes, P0300 is the only code.

Also, thanks for the part number. I wasn't sure which size Felpro gaskets I need; they list 3 part numbers for the 4.3, and Mallory lists one for the Marine intake. I wanted Felpro, but was wondering if the marine intake is a different metal that would benefit from the marine gaskets. I know the Felpros make the special kind, which I agree should be the way to go. Thanks.

Dielectric grease on the inside, is just as important as the outside. The terminals are made of aluminum, (only silver and copper are better conductors of electricity). The problem with aluminum is that it corrodes causing resistance to increase. The dielectric grease retards the corrosion process. When spark jumps from the rotor to the cap, the grease will turn to a white crystalized stuff that looks similar to corrosion, don't scrape it off.

Your Accel cap has brass terminals... good selling point cause they look pretty. I could care less how it looks, I'd rather have a cap that works better.

I checked the fuel pressure. Good. ~62 at prime, ~58 norm, bleed-down @ 10 minutes is under 10psi, but I forget exactly what. Minor leak somewhere apparently, but not the cause of my problem.

I have a new ECU hooked up to ensure the flashing of my last ECU didn't mess anything up. Problem is that the car won't start with the new ECU. I have read elsewhere of this problem, but usually the no-start turns out to be non-ECU related. I had the car started before switching the ECU, so that is not the issue. I guess I will plug the old ECU back up and see what happens.

I found out that the backfiring I was experiencing was from two spark plug wires being crossed on the passenger side (I forget which cylinders - they were beside each other). Whoops. I was so relieved to have found that issue; however, I still get a misfire.

To test the cap and rotor, I bought new ones, which I am about to install.

I have a new-new distributor which I will install when I get a phase scanner tool. I stripped one of the screw holes where the cap screws on and so it bothers me enough that I am going to replace the distributor.

Beyond that I guess I can test the resistance in each individual plug wire, though I don't expect to find anything amiss with the brand new MSD wires. Likewise, my MSD 6-AL ignition should be powerful enough to overcome any minor issues. For testing, however, I disconnect the MSD ignition.

The only remaining component is the rag that, although it apparently fell out the end of the intake tube on start-up instead of getting sucked in, is the only thing left. It seemed like the rag had fallen out, but something as weird as a rag in the intake would be tantamount to the bizarre issues I am experiencing.

After correcting the crossed plug wires, the truck runs smooth above idle, but wants to stall when returning to idle. Would this hold true if there were a rag in the engine? I'm thinking a rag would be noticeable at higher RPM also, when it is sucking in more air. And if the small rag is past the upper intake plenum, wouldn't it be caught in the lower plenum at one of the cylinders? If so, wouldn't I get a misfire on that cylinder and not just a random misfire? It would help if I knew how the misfire detection system works -- about to research.

I have a scope now to look into the intake, but I didn't see anything obvious when I looked with a light and mirror, so I haven't used the scope yet. Logic tells me the rag I found was likely the rag that was in the intake tube, but I can't figure out what else could be causing this weird problem that occurred out of nowhere on a formerly perfect-running engine.

Right now, my best bet is the distributor cap, though the one on the truck ran fine on my last Blazer and I only put about 2 months on it. Moreover, I lightly sanded all the terminals and didn't notice and cracks or streaks.

Key on, engine off, fuel pump running pressure of 62psi is fine. As long as pressure was above 55psi at the 10 minute mark, leakdown is fine.

Swapping PCM's is a problem with this engine. If the crankshaft position sensor and or timing cover are moved, removed, replaced or disturbed, OR the PCM is replaced, the crankshaft position sensor relearn must be performed. The crankshaft position sensor creates a magnetic field that is directed toward a 3X tone ring on the crankshaft, and the field on each sensor is unique. The crankshaft position sensor relearn must be performed so the PCM can learn exactly where the magnetic field is directed. The relearn data is stored in the PCM until the next relearn overwrites it. If the relearn is not done, the PCM will use the data from the last relearn, which is incorrect. The engine will start and run with incorrect data, but not properly. Every calculation and adjustment that the PCM makes is based on crankshaft position sensor data, it's got to be correct.

The PCM uses crankshaft position sensor data, along with camshaft position sensor data, to detect, and accurately identify cylinder misfire. The camshaft position sensor is located in the distributor. The reading is viewed on a scan tool as "camshaft retard". When it is adjusted correctly, the sensor is aligned with the camshaft and the rotor segment is aligned with the terminals in the distributor cap when the ignition coil fires. This reduces/eliminates crossfire inside the cap and increases the longevity of the cap. The adjustment is done by slightly rotating the distributor and watching the scan tool. The throttle must be snapped to 2Krpm and back to idle before the reading is accurate. Desired spec is zero degrees with a tolerance of + or- 2 degrees. Camshaft retard needs to be checked/adjusted every time the distributor is installed, or disturbed. The farther off the adjustment is, the more crossfire in the cap. The accuracy of misfire detection and cylinder identification decreases as well.

Oh, I see. I suppose I will pop the old ECU in and go from there. I am assuming the ECU only needs cam offset adjustment and therefore the new dist will need to still be installed into the same gear alignment as the old one. In other words, it is still very important to install the new dist in the same position as the old one, then simply adjust a few degrees. If this is the case, and with dielectric grease on the cap terminals, will the truck be able to start, even misaligned, so that I may read a scanner and tune the "camshaft retard" by slightly turning the dist one way or the other? If not, how do I get the adjustment close enough to start the vehicle, other than trial and error?

As for the new ECU, I believe it is being rejected by the security feature somehow, as I have read people mention. Although it used to sound like it had no fuel (since installing the new ECU), now the starter is the only thing I hear (no engagement, just clicking (battery good)). I would like to be able to test this new ECU, since it is already installed, but if it is too much a hassle, I'll just work on reinstalling the original ECU. However, I expect I will still encounter the same issue, and I do not understand how/why the security feature disables the car, nor how to bypass it. Maybe it will recognize the old ECU? I don't know how that works, do you?

You mention that the ECU needs to "learn" the new crankshaft position data. Does this happen automatically, in contrast with the manual adjustment of the camshaft sensor? If so, how can it learn the data if it doesn't know enough data to run to engine to monitor additional data? Am I missing something? How does the ECU learn the crank position? Furthermore, what do I need to do to facilitate the process?

And finally, how, as you suggest, can things become permanently out of sync after disturbing the dist during, say, a routine rotor replacement? As long as it stays in the same gear (which it would, of course) and if the base/assembly is not loosened or rotated, how could the camshaft sensor become permanently misaligned/maladjusted (and in need of readjustment)?

Thank you so much for your wealth of knowledge. You are a great help!

I just checked the manual and I don't think camshaft retard is a parameter that my scan tool monitors, although it seems to have everything else, such as Ign timing. Do you know if the couple-hundred dollar units at the local store should do the trick, or do I need a high-end scanner?

If you want to do it yourself:

Cheap scanner hardware:
OBDII Bluetooth Car Diagnostic Cable - Black + Blue + Orange (DC 12V) - Free Shipping - DealExtreme

Scan XL software:
PPE Online Store

Scan XL GM Enhanced software add on needed to read CMP retard:
PPE Online Store