[n2ogpsj462]

2000 blazer with random misfire code. Ive done plugs, wires, cap and rotor (which were JUNK by the way), coil and fuel spider. I only get the misfire on the highway at or above 65mph. Regular driving off the highway and I never get misfires.Using my Solus, I get mass amounts of misfires in 1 cylinder on opposite banks. I forget which ones but they shared nothing. Basically Ive given up trying to figure this out for the last 6 months until recently. My heater core clogged up so I flushed it. removed the thermostat and having running a solution thru it for the last 2 weeks. Now the temp doesn't go above 130 and guess what, no more misfires. When I bought the truck I noticed it ran very cool (160ish in the summer) I put the factory 195 in because I was told mpg would suffer if it didn't get to proper temp. I never put 2 and 2 together but that is around when I started getting misfires. it doesn't overheat so headgaskets are out. any ideas?? fuel pressure is right. About 6psi short at idle but fine driving

[mechanic17]

If it really is related to temp, I've seen both GM and Honda vehicles where the ECM (spark controller, or whatever they're calling it) came with thermal paste because it was bolted to the cylinder head. These overheat and function erratically or fail.

I recently dealt with a 2000 coil-on-plug Honda that was throwing random misfire codes, but it turned out just one bad coil fixed everything. Sometimes, one code, sometimes four or five. So my OBDII scanner was nearly worthless there. But if you've got a regular distributor and changed the coil, I'd look for the ECM (it may have fins on it for heat dissipation too.)

You'll want that temp higher, I think, for several reasons, but a 180 T-stat should do.

FWIW. Good luck!

[n2ogpsj462]

My ECM in fenderwell mounted coincidentally underneath my overflow tank which is obviously running cooler like everything else. I dismissed the ecm early on only because a shop around the corner from mine (I do transmissions so I don't see a lot of this) said the only real ecm issues they've seen relating to misfire issues have been ghost codes. Meaning its throwing misfire codes but not misfiring. I can def feel it misfiring. I manage to get it thru inspection by staying off the highway for a few days but am flushing the system this week and putting a thermostat back in obviously so that I have heat. maybe ill play with drilling some holes to keep it where it wont misfire unless someone can say they've seen this before. Everyone I ask says headgaskets which is just ridiculous considering it would overheat regardless of having a stat in there

[SRR]

Use a detector to see if your getting combustion gases in the coolant.

[Txbobcat]

I had that random misfire on my 4.3 chevy truck. Nobody could find anything wrong. I finally threw some seafoam down the brake booster vacuum line and it cleared up for a few months and then had to do it again. Everytime it does it now which is about every 4 months I throw the seafoam at it again.

[skipp65]

Could be an exhaust valve hanging up with the higher heat,valve stem gets tight in the guide under higher temps. Check compression when it's hot.

[n2ogpsj462]

truck runs phenomenal off the highway. never comes close to overheating even with a/c on in the hottest summer days. If a valve where hanging, why would it only hang on the highway above 65mph? Rpms are about 19-2000 cruising. def worse when going up an incline. Ive gone thru all this in my head and come up with absolutely nothing until I coincidentally pulled the thermostat to flush the system.

[Lightfoot]

Maybe changing to a hotter/colder set of spark plugs would do the trick.
It would take a bit of research to determine what you're running now, verses what other heat ranges would fit your 4.3.

I must admit, I don't know what defines a 'hot plug' or a 'cold plug'.

[Ventura455]

First thing is hang a scanner on it and look and the cam retard timing. It should read 0 on the scanner while running.If it reads anything but 0
I would pull the dist cap and check for shaft slop. grab on to distrubitor rotor and try to spin in back and forth. If you can move it back and forth the distrubitor gear is shot.If the gear is shot just put another distrubitor in it. If the gear is good and the cam retard reads anything but 0 then remove the distrubitor hold down and grind on it til you are able to turn the distrubitor. Once you can turn the distrubitor you can set the cam retart to 0 and lock down the distrubitor, reset the engine light and you should be good to go. The dist hold down is designed from the factory to not allow the distrubitor to be adjusted.
Steve

[skipp65]

Have you checked fuel pressure running down the highway while it's misfiring? Those 4.3's are very picky about fuel pressure,have you checked fuel filter?

[n2ogpsj462]

Hey ventura, I read about the distributer woes when I was trying to figure this out last yr. rotor was tight and cam was zero so I ruled it out. I think I may have a bad ecm. its mounted directly on top of the reservoir. I installed the 180 stat and the truck runs dead on 180 but I left the hose capped that leads over there so that water is staying whatever bay temp is. Im going to hook the hose back up to verify but I think I know the answer. Funny this is the truck runs phenomenal otherwise without any other codes or concerns. every other 4.3 ive owned would have a slight miss at idle or other odd issies but not this one. Ive driven this truck for 8 months like this and every single time on the highway ( and I mean 100% of the time) it would misfire like crazy above 65. Can I grab a yard ecm or and program it to accept my key? I just got the updates on my Solus that allows me lots of calibrations now but this is kinda outta my field

[Ventura455]

I highly doubt that the ECM is the problem as I work on alot of these vortec type injections systems and have never replaced an ECM for a misfire problem. Have you replaced the lower intake gaskets? Those are a big problem. I use the felpro steel intake gaskets instead of the plastic crap from the dealer. What other cylinders do you have misfires on? When you replaced the crab unit did you use a factory replacement or aftermarket as it is posible that #1 injector is bad even though it is new. Its a pain in the ass but you can swap that injector tube to another hole and see if the misfire moves to the swaped hole.
Steve

[nashcar]

Had the same with my 2002 s10. I used 2 bottles of Techtron fuel system cleaner from Walmart, no more misfires. Techtron was the only one recommended to me. I also fill the tank with ethanol free every 1,000 miles.

[n2ogpsj462]

I installed the updated style with injectors. regardless I had the same problem the first trip down the highway. I'm honestly not pursuing this issue until its shows itself again. so far with the 180 stat in there I have zero issues. Makes no sense which is why I threw it out there to see if someone coincidently came across it. I have a parts account with my local chevy dealer and ran it by the service writer (older guy who claims to have seen it all) and nada. chances are no one ever put 2 and 2 together or had to remove a thermostat for any reason during their diagnosis.

[PunchT37]

Have you checked the coolant sensor data against a known thermometer? I`d like to see if the computer sees the temp. that the engine actually is.

In other words, not the sending unit for the guage, but the 2 wire coolant sensor? I believe they are yellow and black wires.

[Stuckinda60s]

Originally Posted by PunchT37

Have you checked the coolant sensor data against a known thermometer? I`d like to see if the computer sees the temp. that the engine actually is.

In other words, not the sending unit for the guage, but the 2 wire coolant sensor? I believe they are yellow and black wires.

BINGO! I was reading through this thread and thinking this might be the trouble. My other choice would be the fuel pressure regulator. You might also have both problems and they're affecting each other. Here's a chart showing the resistances of the sensor at different temps. Simply remove the connector and check the resistance before you run the engine. Then, run the engine until it reaches operating temp and check it again.

Coolant sensor approximate resistance specifications:
177 ohms @ 212 deg. F. or 100 deg. C.
241 ohms @ 194 deg. F. or 90 deg. C.
332 ohms @ 176 deg. F. or 80 deg. C.
467 ohms @ 158 deg. F. or 70 deg. C.
667 ohms @ 140 deg. F. or 60 deg. C.
973 ohms @ 122 deg. F. or 50 deg. C.
1188 ohms @ 113 deg. F. or45 deg. C.
1459 ohms @ 104 deg. F. or 40 deg. C.
1802 ohms @ 95 deg. F. or 35 deg. C.
2238 ohms @ 86 deg. F. or 30 deg. C.
2796 ohms @ 77 deg. F. or 25 deg. C.
3520 ohms @ 68 deg. F. or 20 deg. C.
4450 ohms @ 59 deg. F. or 15 deg. C.
5670 ohms @ 50 deg. F. or 10 deg. C.
7280 ohms @ 41 deg. F. or 5 deg. C.
9420 ohms @ 32 deg. F. or 0 deg. C.
12300 ohms @ 23 deg. F. or -5 deg. C.
16180 ohms @ 14 deg. F. or -10 deg. C.
21450 ohms @ 5 deg. F. or -15 deg. C.
28680 ohms @ -4 deg. F. or -20 deg. C.
52700 ohms @ -22 deg. F. or -30 deg. C.
100700 ohms @ -40 deg. F. or - 40 deg. C

Living in PA with our wide temp swings, you really need that sensor. So you can't just use a resistor across the sensor and fool the ECU into thinking the engine is operating at a constant temp. If the engine thinks it's running at low temperatures, it goes into open loop and goes rich. This is good for power, but bad for emissions and economy. I'm thinking that the sensor might be reading low (high temp), affecting timing and fuel and causing a misfire condition. It might be exaggerated by the low pressure from the fuel pump.

[n2ogpsj462]

That's why I initially put the factory 190 stat in when I bought the truck. Ive had a few of these and they all ran right in the middle.It was running at 160 and I knew I would get better mpg by simply changing it back to factory. Right now its freezing here and the truck runs at 160-165 and heat works well enough so im honestly not touching it and driving myself insane over a few mpgs. If the issue presents itself again then ill pursue it further.