'96 252 454 Magnum performance benchmark

[akorcovelos]

Idle is still a tad lower than I would like as well. Once warmed up it idles okay, but cold it will stall if I shift too fast. I’ve gotten several compliments on how good it sounds with the tune, and embarrassed a nice fellow in a Baja Outlaw yesterday. Gotta love the sleeper status of a Cobalt with Captains Call.

[akorcovelos]

ok, so I've been reading a LOT on the magnum MPI motors and I have a theory I'm going to check today or tomorrow. I wanted to check base timing so I set the ECU to diagnostic mode with the paperclip method, but I quicly found that I couldn't get the engine to hold at 1200-1500 RPM to get an accurate reading on the timing light. It would surge from 800-2000 RPM even at full operating temp, no way to even hold it at 1000 RPM.

That made me think about how I've had to start the boat since day 1, by giving it more throttle than the high idle setting on the controls, then feather it down to idle, even on the stock tune. Last time I was out I noticed that there is ZERO RPM change from neutral to the high idle setting.

Now I'm thinking I've either got a bad IAC, a bad/mis-adjusted TPS, or both. I'm going to see if there are any codes stored, maybe in both ECUs.

If the TPS is off, and not reading the change from neutral to high idle, that would mean it's not seeing WOT either, and I believe this EFI is old enough that it's a 3 position TPS, idle, mid-range, and WOT, vs a linear voltage TPS, right?

Thoughts?

[cmattj]

ok, so I've been reading a LOT on the magnum MPI motors and I have a theory I'm going to check today or tomorrow. I wanted to check base timing so I set the ECU to diagnostic mode with the paperclip method, but I quicly found that I couldn't get the engine to hold at 1200-1500 RPM to get an accurate reading on the timing light. It would surge from 800-2000 RPM even at full operating temp, no way to even hold it at 1000 RPM.

That made me think about how I've had to start the boat since day 1, by giving it more throttle than the high idle setting on the controls, then feather it down to idle, even on the stock tune. Last time I was out I noticed that there is ZERO RPM change from neutral to the high idle setting.

Now I'm thinking I've either got a bad IAC, a bad/mis-adjusted TPS, or both. I'm going to see if there are any codes stored, maybe in both ECUs.

If the TPS is off, and not reading the change from neutral to high idle, that would mean it's not seeing WOT either, and I believe this EFI is old enough that it's a 3 position TPS, idle, mid-range, and WOT, vs a linear voltage TPS, right?

Thoughts?

Interesting as it almost seems as there is 2 issues.
First im wondering if that year has a shift interrupt that may be preventing start up you say its done since day one.

How I would test it is disconnect the cable to the t-body and see if it starts ( 2 people ) by lightly moving the tbody and leaving the shifter in neutral. If it starts then we can way out the shift interrupt. Now we target T body cleaning, and possible see if a backside wear grove is forming (carbon or wear) allowing a different setting.
Sure its possible about IAC and TPS but Im curious to see what the actual running readings would be vs the commanded positions first. As they aren't big$$ we could throw them at it after we check the first items.
Im looking for a Demon 750cfm, and a Aluminum air gap manifold. BB has connections

Any keep us posted pal!

[Big Block Power]

What kind of diag plug does your boat have? Do you have a scanner? You should of course never have to give it gas to start it which I know you know. Also my engine doesn't idle any higher when it's cold.

[akorcovelos]

Interesting as it almost seems as there is 2 issues.
First im wondering if that year has a shift interrupt that may be preventing start up you say its done since day one.

How I would test it is disconnect the cable to the t-body and see if it starts ( 2 people ) by lightly moving the tbody and leaving the shifter in neutral. If it starts then we can way out the shift interrupt. Now we target T body cleaning, and possible see if a backside wear grove is forming (carbon or wear) allowing a different setting.
Sure its possible about IAC and TPS but Im curious to see what the actual running readings would be vs the commanded positions first. As they aren't big$$ we could throw them at it after we check the first items.
Im looking for a Demon 750cfm, and a Aluminum air gap manifold. BB has connections

Any keep us posted pal!

Interesting you mention the shift interrupt, because the dealer installed new shift cables before I bought the boat, I wonder if an improperly adjusted shift cable could cause my symptoms? I know it’s anecdotal, but it shifts on and out of both gears perfectly, at the perfect throttle position. If the throttle cable isn’t adjusted right that would explain not getting high idle until past the high idle position. Am I talking crazy now?

I planned on pulling the throttle body and cleaning it after I pull any codes that are stored, but I already took the flame arrestor off and wiped the out butterfly’s and throat down. It wasn’t dirty at all, so I doubt it’s got a carbon ring built up or anything.

I should mention that once it fully warmed up it fires right up and falls into idle, but when it’s cold it will stall when I go to shift from rev to fwd. From what I’ve been reading the IAC can present these symptoms when they fail.

Also, to be more clear, on cold start I’m just pushing the throttle a couple inches past the high idle position, not pumping it like a carb. That’s what makes me suspect the TPS, because it not getting out of the idle table until after the high idle position. What I haven’t been able to find out is if a faulty IAC can also cause this symptom.

Sometimes I wish I had good ol Holly sitting on a single plane Edlebrock high rise intake!

[akorcovelos]

What kind of diag plug does your boat have? Do you have a scanner? You should of course never have to give it gas to start it which I know you know. Also my engine doesn't idle any higher when it's cold.

It’s got the 10 pin Merc diagnostic plug. Unfortunately I don’t have a marine scanner, but it does have flash codes stored that I can read with a DMM.

I thought it should jump up to 1100 rpm and step down to ~800rpm steady with the throttle in the neutral position?

[Big Block Power]

Ok maybe we have 2 Different ideas of what a fast cold idle is. I guess I'm thinking of a carburated fast idle as it stays high for a period of time. A idle that goes up then comes to its proper speed is a diff thing to me.oh when my shift interupt cable was bad my engine wouldn't start then it would later.

[akorcovelos]

Ok maybe we have 2 Different ideas of what a fast cold idle is. I guess I'm thinking of a carburated fast idle as it stays high for a period of time. A idle that goes up then comes to its proper speed is a diff thing to me.oh when my shift interupt cable was bad my engine wouldn't start then it would later.

I know, you’re talking more about a choked carb that holds high idle until it’s warm enough to hold normal idle. What I’m talking about is like when you jump in your car and just crank it over, it should jump up to a high idle and the slowly over a minute or so fall into normal idle in small steps. I assume the MPI has a similar cold start enrichment table.

[Big Block Power]

Ok maybe we have 2 Different ideas of what a fast cold idle is. I guess I'm thinking of a carburated fast idle as it stays high for a period of time. A idle that goes up then comes to its proper speed is a diff thing to me.oh when my shift interupt cable was bad my engine wouldn't start then it would later.

I know, you’re talking more about a choked carb that holds high idle until it’s warm enough to hold normal idle. What I’m talking about is like when you jump in your car and just crank it over, it should jump up to a high idle and the slowly over a minute or so fall into normal idle in small steps. I assume the MPI has a similar cold start enrichment table.

Honestly this one I have now is not that dramatic. I wish it was. The program is not like a car. Mine might go right to 750 maybe then to 675. Or it will just go to 675. I had him bump that from 650 to 675 this year because of that too. Matt what do yours do? My 8.1gi was tuned a bit diff but not much.

[cmattj]

Mine idles 650 but I’ll check the command idle. I do see a warm cruise speed idle return of 750 but drops within 3-5 seconds back down.
Ak may need a scanner to read counts. Then set his timing as that can push a iac around trying to adjust itself. ( especially if timing is retarded).
It’s the start that has me ?? as a typical iac going bad or sticking does an idle stall then reset and starts fine with no throttle input.
It’s as if his cable is over adjusted and playing havoc. Let’s see if removing the cables change anything. Then we’ll spend $$.

[akorcovelos]

Yep, the more I think about it the more I’m leaning towards cable adjustment. Seeing as the dealer replaced part of the controls and installed new cables, and nothing else sensor wise that I’m aware of it seems the most logical answer.

I’ll try and get out to pull some codes today, and I’ll pull the flame arrestor again and see if the throttle plates are fully opening with the controls at WOT.

[akorcovelos]

Well, I didn’t get enough time at the boat to try firing it up, but I verified there are no codes stored in the new ecu, didn’t swap them yet, and the throttle plates are fully opening at WOT. The IAC tube looks pretty dirty, especially at the very bottom, so I think removal and good good cleaning is next.

I’ve found mixed info online for the test values for the tps, anyone know offhand? Either way I’ll do a continuity test to make sure it’s hitting WOT with the butterflies fully opened. I’ll give the spark arrestor a good cleaning too.

[cmattj]

Well, I didn’t get enough time at the boat to try firing it up, but I verified there are no codes stored in the new ecu, didn’t swap them yet, and the throttle plates are fully opening at WOT. The IAC tube looks pretty dirty, especially at the very bottom, so I think removal and good good cleaning is next.

I’ve found mixed info online for the test values for the tps, anyone know offhand? Either way I’ll do a continuity test to make sure it’s hitting WOT with the butterflies fully opened. I’ll give the spark arrestor a good cleaning too.

TPS on 5 volt reference in our GM stuff at idle is usually .4 at idle and WOT should see 4.5 ish
I would think it’s similar in the GM marine world.
Yes clean the iac but keep in mind after cleaning they can rust up and get twitchey. We just change them because we spend others peoples $$.

[Big Block Power]

Mine is at.. 31v iac counts 120 not running at key on. I had a hard time finding the correct tps trading when I was looking they freaky don't post it. But I'm with Matt where mine is too.. 50
I'm not sure if my diag cable would work on your connector. As mine and Matt's were the same. So we are hooked up.
Take a picture of yours. I'm sure I could send you mine too try with the software. But the session isn't going to be around much longer and you can do it next year too.

[akorcovelos]

Mine is at.. 31v iac counts 120 not running at key on. I had a hard time finding the correct tps trading when I was looking they freaky don't post it. But I'm with Matt where mine is too.. 50
I'm not sure if my diag cable would work on your connector. As mine and Matt's were the same. So we are hooked up.
Take a picture of yours. I'm sure I could send you mine too try with the software. But the session isn't going to be around much longer and you can do it next year too.

This is the diag plug I have:


If I’m pulling the throttle body I’ll just replace the IAC. I need to verify by engine serial but I think this is the right one:
https://leadersrpmshop.com/mercruiser-n ... v9EALw_wcB

I’ll take it apart Sunday and see how it all looks. We got 5” of rain in 2 days so the lakes are all no wake, and probably will be for a few days so it’s good timing I guess...