Recent weirdness

[liquify33]

Long version: skip below for the question.
Late last year I did the intake manifold gasket. Couldn’t start afterwards and chased it down to crank sensor, the plug was left in the coolant drain bucket and the sensor itself was bad. Cleaned it off, sprayed it good with contact cleaner and had a couple hiccups where unplugging and re-plugging that connection got me running right. Replaced: ICM, coil, distributor (broke on the 30th reinstall), spider upgrade, plugs and wires. Mostly Delco parts (tho the new plugs are .040 instead of .060).

In December the transmission wasn’t acting right. Had the filter/fluid change done but the truck still didn’t like high rpms in 3rd and shook violently. Threw no codes. Lately it started shaking at 60. Back off or speed up and mostly ok. Got new tires (mine were dry rotted). Ran premium gas with B12 cleaner in the tank.

Got a cheap scanner and started monitoring closer. It was misfiring at 60ish then moved up to 70 (60 now good). P0300 and P0305 if I left it in the angry zone long enough. Several hundred times according to scanner, felt like I was driving on square tires. Wires are tight on plugs and cap.

Yesterday the crank sensor connector (harness not sensor) finally gave out. In the space of 10 miles I had to get under and jiggle the wires 3 times until I could hear the fuel pump. Got a new plug I’ve been waiting on nicer weather to install and spliced it in. It’s a hack job I did in a Walmart parking in 40degree rain. But it runs again, will clean it up soon. Truck runs nice but still misfires at highway speed. 70 is now OK, 75+ still angry.


Ran the gas down, added Seafom at around 10gals remaining.Ran that as low as I dared but no misfire now. Refilled and still good. Double checked air filter connections and hit that reset button (WTF IS THAT GUY??) on the little valve.


Questions/theory:
The transmission issue when gunning on the highway seems to have been fuel/ignition related. Outside temps have not changed significantly.

Bad gas? Fouled injector?

 

[swathdiver]

Still have cats? Can you see fuel pressure? How about fuel trims?

 

[OR VietVet]

Maybe, the injector(s) dirty and the Seafoam cleaned it out. The more load on the engine, higher RPM's, the more pronounced the clog can be. Drive it like you stole it now and see what happens.

 

[liquify33]

Still have cats? Can you see fuel pressure? How about fuel trims?

Yes to cats. Pressure is fine in the driveway and holds. I’ll have to log the fuel trims, hard to see while doing 75!

I’m struggling as to what would cause misfires at speed/load with rpm’s only 2k. Problem *seems* to be clear for now, worried about it coming back next time I need to get from 50-70 ASAP in Houston.

Also, this is the best the truck has run since I’ve had it the last 13 months. That little harness was definitely part of the issue.

 

[liquify33]

Drive it like you stole it now and see what happens.

I gave it a couple “Italian tuneups” on the way home and she was quite happy.

 

[OR VietVet]

I used to run shops and years ago I had a tech or two that thought part of a tune was to get all parts changed and then rev that engine several times to 3k-4k rpm's. I called it an "Arkansas Tune-Up". I told them they can monitor rpm from idle to 2k to 2500 rpm and a snap throttle or two but repeated high rpm roars were not needed. Especially sitting still. I did all the road tests. They learned or was gone. To me, that is abuse unless it is a performance/competition vehicle.

 

[swathdiver]

Yes to cats. Pressure is fine in the driveway and holds. I’ll have to log the fuel trims, hard to see while doing 75!

I’m struggling as to what would cause misfires at speed/load with rpm’s only 2k. Problem *seems* to be clear for now, worried about it coming back next time I need to get from 50-70 ASAP in Houston.

Also, this is the best the truck has run since I’ve had it the last 13 months. That little harness was definitely part of the issue.

We've seen failing cats cause misfires due to too much backpressure. Have to see what the fuel pressure is during those misfires, if the pump is able to maintain pressure and volume under load. Trims might offer some more clues.

 

[iamdub]

These things eat distributor caps and rotors. That's a quick and easy check. If replacing it, get AC Delco or Borg Warner. DO NOT GET STORE BRAND (ESPECIALLY AUTOZONE) IGNITION OR ELECTRONIC PARTS.

The tighter gap in the plugs could actually cause problems as well.

 

[liquify33]

We've seen failing cats cause misfires due to too much backpressure. Have to see what the fuel pressure is during those misfires, if the pump is able to maintain pressure and volume under load. Trims might offer some more clues.

That had crossed my mind also but I haven’t heard a rattle or had the funky smell from failed cats I’ve had previously. I’ll poke around the app (DashCmd) and see how to log.
As a side note it also measures cam offest for those who change distributors for an extra $10.

 

[liquify33]

These things eat distributor caps and rotors. That's a quick and easy check. If replacing it, get AC Delco or Borg Warner. DO NOT GET STORE BRAND (ESPECIALLY AUTOZONE) IGNITION OR ELECTRONIC PARTS.

The tighter gap in the plugs could actually cause problems as well.

I’m really tempted to re-install my Delco ICM and coil. Has anyone tried the Davis Unified “performance” coil? Longer spark dwell is what they advertise.
ETA: I moved the distributor lower on the list to check as it happens only at high speed. Issue did not follow RPMs.

 

[OR VietVet]

The gap is important because that can cause too weak or too strong of a spark and cause running problems that can lead to damaged ignition components and parts like cat converters.

 

[iamdub]

I’m really tempted to re-install my Delco ICM and coil. Has anyone tried the Davis Unified “performance” coil? Longer spark dwell is what they advertise.

I'm kinda iffy about lengthening the spark time on dizzy ignitions. I had a MSD on my 4.3 just cuz it was laying around. I was burning through caps and rotors much faster than I should have. Then I learned that the extra spark time/multiple sparks was like multiplying the usage of the conductors in the cap and rotor. To put arbitrary numbers on it as an example: If the parts are rated for 1,000,000 sparks before they're worn, having sparks twice as long or twice the number of sparks would make it reach that 1,000,000 spark life limit twice as soon. If the MSD did make any power, it wasn't enough for me to tell. IMO, stock motor is fine with stock parts unless there's a known design flaw. You're already shorting yourself spark size with the 33% smaller gap.

 

[liquify33]

That has added to my confusion as supposedly GM changed the spec on the iridium plug for these to .040. They were all gapped the same when I received them, had also read not to change gaps on iridium plugs.

These are the plugs I used:
ACDelco 41-993 Professional Iridium Spark Plug (Pack of 4) https://www.amazon.com/dp/B075XWYMYD/ref=cm_sw_r_cp_apip_RaRrCHHrTAKKC

 

[OR VietVet]

From RA: https://www.rockauto.com/en/moreinfo.php?pk=1021537&cc=1353721&jsn=2256


Where it says gap size it says .059. Basically .060 gap.


I found as I read thru the questions people ask, at the Amazon link, about the plugs that someone said they had "HEARD" that the plugs should be gapped at .040 instead of .060 and was asking what they buyer had to say about that. His answer is this:

"I have a 2002 Chevy S10 pickup with a 4.3 V6 engine. It calls for .60 gap for the spark plugs. I bought the AC Delco 41-993 and gaped them to .60. They have been in my truck for a year now.
I haven't had any problems with my engine."


I have never heard of changing the gap to .040. Out of the box the iridium plug should not be regapped. Not good on the center or ground electrodes to move them and especially change the gap by 33%. Needs .060 gap on a new set of plugs.

 

[liquify33]

From RA: https://www.rockauto.com/en/moreinfo.php?pk=1021537&cc=1353721&jsn=2256


Where it says gap size it says .059. Basically .060 gap.


I found as I read thru the questions people ask, at the Amazon link, about the plugs that someone said they had "HEARD" that the plugs should be gapped at .040 instead of .060 and was asking what they buyer had to say about that. His answer is this:

"I have a 2002 Chevy S10 pickup with a 4.3 V6 engine. It calls for .60 gap for the spark plugs. I bought the AC Delco 41-993 and gaped them to .60. They have been in my truck for a year now.
I haven't had any problems with my engine."


I have never heard of changing the gap to .040. Out of the box the iridium plug should not be regapped. Not good on the center or ground electrodes to move them and especially change the gap by 33%. Needs .060 gap on a new set of plugs.

If it comes back I’ll try some platinums at .060 and re-test.

Googling this a couple months ago when I did the work on the intake it seems like the confusion started around 2010 and much back and forth on 40 or 60.

 

[OR VietVet]

I get it. I tend to stick with what I know absolutely for sure and what I don't know I hunt till I am blue in the face to know for sure.

 

[liquify33]

I get it. I tend to stick with what I know absolutely for sure and what I don't know I hunt till I am blue in the face to know for sure.

Looks like I may have another plug vote. Same codes.

 

[iamdub]

I didn't know you had Iridiums. That may make a difference, I don't know.

 

[east302]

Got a cheap scanner and started monitoring closer. It was misfiring at 60ish then moved up to 70 (60 now good). P0300 and P0305 if I left it in the angry zone long enough. Several hundred times according to scanner, felt like I was driving on square tires. Wires are tight on plugs and cap.

Hopefully it’s fixed. If it comes back, the P0305 code points to cylinder #5. The scanner app should have a PID for misfires by cylinder number, so it would be worth checking that to see if it is fixed on multiple cylinders or just one.

Edit: if it comes back and is on one cylinder, try swapping the plug with another cylinder and see if the misfire follows.

 

[OR VietVet]

At RA whether they are copper, platinum, double platinum or iridium, they all call for .060 gap.