4.3 V6 ECM on a V8: which way should I go?

**1project2many** · 08-16-2021

One of the hard parts about tuning EFI is that you can't really see what's going on. The surge is probably due to several factors acting together. If the engine was idling a little rough due to the cam it's possible that the ecm saw a high MAP sensor reading. High MAP = low vacuum and to the ecm this means the engine is loading up. So the ecm will add spark, then fuel, then IAC to compensate for the load. But... there's no load so the engine rpm increases a bunch. The ecm sees the rpm is too high so it pulls spark advance and possibly reduces fuel and IAC. Now the engine slows down a bunch. And if it slows down too much the ecm thinks it's stalling. So it quickly increases spark advance and throws in a shot of fuel. This causes rpm to increase quickly. Wash, rinse, repeat.

**DOC327** · 08-16-2021

stew86 and 1p2m: Ok, so I went back in the shop today and re-did and re-checked my minimum air rate and TPS as per the older post by EagleMark. The surge is the same with both of the 1227747 ECMs. No different. I plug that old 4.3 V6 ECM in 16144288 and the idle is excellent, as long as I bump up the RPM to compensate for the 25% difference the 4.3 ECM thinks the engine is at. Of course then the 4.3 ECM eventually closes the IAC after it warms up and then my low speed drivability problems come back. 1p2m, your above explanation sounds plausible, but I can't help but feel that both of those 1227747 ECMs I've got are junk, considering that the 4.3 ECM 16144288 maintains some semblance of normalcy. The second 1227747 ECM I picked up Friday came with a 30 day warranty so I'm going to go that route and see if I can get another one: roll the dice. 1p2m, I tried the recording feature on that SLOWTC2000: you were right, pretty useless. I continue to read up on the data logging direction you're promoting, but until I can find a 1227747 that lets my engine run at least as good as with the 16144288 then I might consider going deeper down the rabbit hole. Right now I don't think I've got anything that would be worth recording. FYI, I checked my injectors, and I've got V8 injectors in the throttle body. Also, question, are there other ECMs I could use, like from a full size V8 Caprice/Roadmaster (16136965) that would work as well as the 1227747? The salvage yard I'm working with has a pretty good inventory so I figure I might broaden my ECM search prospects. Thanks Guys for you help so far, I'm learning! Dave/DOC

**tayto** · 08-16-2021

'746 but it will be looking for an IAT sensor.

**stew86MCSS396** · 08-16-2021

Originally Posted by tayto

'746 but it will be looking for an IAT sensor.

An 89 9C1 would be equipped with a L98 cam which might be the closest thing to your set up although it does have larger injectors.

What rpm are you trying to set the minimum air/idle to? In a earlier post, you mentioned your idle of 650 rpms. Problem may be that the PROM bin is targeting 500-525 @ 95-100C* and you're trying to set it to a higher rpm. Once the IAC is reconnected, it will target 500-525 rpms by closing the IAC to the point it can't close it anymore and you lose some control-ability. One other thing, do you have a functional park/neutral switch? If not my understanding is to run it open.

**ony** · 08-16-2021

project 1 too many iam glad your still here

**DOC327** · 08-16-2021

Originally Posted by tayto

'746 but it will be looking for an IAT sensor.

Tayto, thanks for the suggestion. I wouldn't be opposed to adding an IAT to the mix. What is the "746" used on? is that a V8 Caprice ecm? Whats the whole number?

**DOC327** · 08-16-2021

Originally Posted by stew86MCSS396

An 89 9C1 would be equipped with a L98 cam which might be the closest thing to your set up although it does have larger injectors.

What rpm are you trying to set the minimum air/idle to? In a earlier post, you mentioned your idle of 650 rpms. Problem may be that the PROM bin is targeting 500-525 @ 95-100C* and you're trying to set it to a higher rpm. Once the IAC is reconnected, it will target 500-525 rpms by closing the IAC to the point it can't close it anymore and you lose some control-ability. One other thing, do you have a functional park/neutral switch? If not my understanding is to run it open.

Stew86: yes I do have a confirmed functional P/N input. And I've fiddled with the minimum idle/air rate both ways depending on which ECM I plugged in. The only decent idle I could get was with that 16144288 V6 ecm. Interesting also that the 16144288 runs so good at highway speeds AND I'm not setting ANY codes! Would you, or anyone else, have ever seen the 1227747 fail like I'm experiencing? I've got two of them doing the same thing: 500-1200 cycling of the idle rpm at 10-15 second intervals. Is that a typical failure for any ECM? Thanks

**stew86MCSS396** · 08-16-2021

Originally Posted by DOC327

Tayto, thanks for the suggestion. I wouldn't be opposed to adding an IAT to the mix. What is the "746" used on? is that a V8 Caprice ecm? Whats the whole number?

http://gearhead-efi.com/Fuel-Injecti...Information-61

**tayto** · 08-16-2021

I don't think your ECU is to blame,it's the calibration. I think the fact it idles on the V6 cal is just lucky. Not much fuel is needed to run at part throttle or light cruise so not surprising it runs OK there. Just went through this with the Qjet I put on my new engine in my truck. It needed work on the idle circuit and WOT. I also can't see the timing map being anywhere near optimal for your engine combo either.

**1project2many** · 08-16-2021

but I can't help but feel that both of those 1227747 ECMs I've got are junk

That's your choice. But I can tell you that surging idle is one of the most common symptoms of mismatched tuning. Heck, I've got that very issue (at times) in my '99 Suburban. I replaced the factory 350 with a short stroke 302 and when conditions are right, it surges at idle under load.

It's often helpful to prevent the ecm from changing timing. The ecm is able to alter timing faster than fuel or IAC and timing changes can have the greatest effect on idle speeds. If you have a "set timing" connector on the tan/black wire then you can disconnect it and see if engine idle speed stabilizes. If you don't have a set timing connector (you may want to install one) you can try placing a jumper across terminal A and B of the ALDL. Don't try to drive this way, but you can start the engine and let it idle for a bit to see if idle is more stable.

Would you, or anyone else, have ever seen the 1227747 fail like I'm experiencing?

Possibly once that I can remember. The surging was very, very rapid and the overall idle speed was very high. The ecm would not communicate with the scantool and the check engine light was illuminated as well. It was very apparent from the moment I started the engine that there was a problem.

As far as junk ecm, these things are usually pretty reliable. I won't say you don't have two failures but chances are low. If they'll exchange the one you got, maybe it will be good to try a third, just to be sure.

**DOC327** · 08-18-2021

Originally Posted by 1project2many

That's your choice. But I can tell you that surging idle is one of the most common symptoms of mismatched tuning. Heck, I've got that very issue (at times) in my '99 Suburban. I replaced the factory 350 with a short stroke 302 and when conditions are right, it surges at idle under load.

It's often helpful to prevent the ecm from changing timing. The ecm is able to alter timing faster than fuel or IAC and timing changes can have the greatest effect on idle speeds. If you have a "set timing" connector on the tan/black wire then you can disconnect it and see if engine idle speed stabilizes. If you don't have a set timing connector (you may want to install one) you can try placing a jumper across terminal A and B of the ALDL. Don't try to drive this way, but you can start the engine and let it idle for a bit to see if idle is more stable.

Possibly once that I can remember. The surging was very, very rapid and the overall idle speed was very high. The ecm would not communicate with the scantool and the check engine light was illuminated as well. It was very apparent from the moment I started the engine that there was a problem.

As far as junk ecm, these things are usually pretty reliable. I won't say you don't have two failures but chances are low. If they'll exchange the one you got, maybe it will be good to try a third, just to be sure.

1p2m: thank you for this response. I'm hoping I can get back to the salvage yard on Thursday and trade in my suspect 1227747 ECM. When I had it on the engine the surging was pretty dramatic and fwiw the surge is gone and the idle is stable when I unplug the set timing connector (timing at 0 degrees). Turned off the key. Then I hooked up the set timing connector and pulled/reconnected the ecm fuse. Upon restart I get the 500-1200 RPM surge (after maybe 30 seconds) and if I let it do this long enough (maybe 2-3 minutes) it seems like the engine loads up and will set a Code 44 (lean). It does communicate with the scanner; the IAC is surging with the rest of the gang. Frustrating. So give me a few days and I'll report back, after Thursday.

Another question? I'll also be looking for a 1228746 at the salvage yard If I can't find another 7747. Looking at the pinouts for 1228746 which is in a 88-92 Caprice/Camaro 5.0/5.7 L03/L05 I see terminal B6 is a VATS (Vehicle Anti Theft) input. Not clear as to what year they started using VATS and if so, is Vats something I can defeat by just unplugging B6, or would it have to be defeated thru programing? Guys, thanks again. I really appreciate all the feedback I'm gettig. This is a great site!! Regards, Doc/Dave

**DOC327** · 08-23-2021

Just an update to anyone who's been following this thread: I've not had time in the last few days to get to the salvage yard to exchange my 1227747; and I'll be looking for a 1228746 ECM instead. Hope to get there later this week. Stay tuned....DOC

**stew86MCSS396** · 08-24-2021

TBH I think the only one that will be beneficial to you is if you can get 1 out of a '89 Caprice ANLU albeit bigger injectors fix w fpr tweak. It'll be mapped for a L98 cam. All others will be mapped for the tiny TBI peanut cam and it'll be like beating a dead horse. http://gearhead-efi.com/Fuel-Injecti...ull=1#post2614

**DOC327** · 08-24-2021

stew86: thanks for the suggestion. Yes I've narrowed my search to B bodies and the 1228746, preferably the the ANLU prom. I hope to get out to the junk yard on Wednesday. TBH, feels like I'm chasing windmills....