MEFI 3 Trouble shoot chart

[HALLETT BOY]

Anybody have a trouble- shoot chart / diagram for a mefi 3 system ? Specifically the IAC circuit ?

 

[Shlbyntro]

A Mefi3 is a pretty stupid simple computer. I love them for that. But it also means they usually wont tell you whats wrong. You have to troubleshoot your engine more like its carbureted vs injected with the only major difference being how they respond to vacuum leaks which is the opposite of a carb.

What are your symptoms?

 

[HALLETT BOY]

A Mefi3 is a pretty stupid simple computer. I love them for that. But it also means they usually wont tell you whats wrong. You have to troubleshoot your engine more like its carbureted vs injected with the only major difference being how they respond to vacuum leaks which is the opposite of a carb.

What are your symptoms?

Can’t get the IAC to respond to scanner request . No idle flair on initial engine start is my original complaint. Engine stalls immediately after start .

 

[4Waters]

Can’t get the IAC to respond to scanner request . No idle flair on initial engine start is my original complaint. Engine stalls immediately after start .

There is an IAC filter, it looks like a piece of scotchbrite, pull that out and see if it improves. Sometimes it's the simplest things.

 

[Shlbyntro]

Very early merc TBI motors did not have a boss for the IAC in the intake manifold. The IAC was there but it did nothing because it went to nowhere. Make sure your motor is not one of these. They are easy to tell because they look like a TBI is bolted to a 2bbl intake manifold via an adapter plate.

Once ruling that out do the iac filter removal/replacement as mentioned by @4Waters.
Beyond that you are really narrowed down to it only being the IAC, wiring harness, or ECM in which case, 99% of the time its the IAC.

It worth mentioning that I always start with checking fuel quality and sparkplugs/compression when there is a runability issue. It's worth ruling it out at the very get go so as not to be chasing your tail down the road.

 

[HALLETT BOY]

Very early merc TBI motors did not have a boss for the IAC in the intake manifold. The IAC was there but it did nothing because it went to nowhere. Make sure your motor is not one of these. They are easy to tell because they look like a TBI is bolted to a 2bbl intake manifold via an adapter plate.

Once ruling that out do the iac filter removal/replacement as mentioned by @4Waters.
Beyond that you are really narrowed down to it only being the IAC, wiring harness, or ECM in which case, 99% of the time its the IAC.

It worth mentioning that I always start with checking fuel quality and sparkplugs/compression when there is a runability issue. It's worth ruling it out at the very get go so as not to be chasing your tail down the road.

This system is on a 565ci with 3.3 whipple . Was working last year . Pintle on IAC is extended all the way closed . Can’t get scan tool to retract , tried new IAC , no change

 

[Shlbyntro]

I've had the older MEFIs not respond to an IAC reset command before ( almost like they dont understand the command from the scanner.)

How are your MAP readings on live data at initial crank when trying to start?

 

[DaveH]

forget about the scan tool. just start it up with the IAC out......you can put a little rubber plug to seal the port (OK if it leaks a little this will just cause a little higher idle speed).

if you don't see the IAC doing anything its most likely a bad IAC.

if you replace it be advised there are many of those 4 wire GM type IAC and they have different pin outs.

you can identify which one you have by taking a multi meter and measuring the resistance across the pins. should see about 50ohm across each pair.

 

[lbhsbz]

This system is on a 565ci with 3.3 whipple . Was working last year . Pintle on IAC is extended all the way closed . Can’t get scan tool to retract , tried new IAC , no change

Check circuit integrity back to the ECM. I prefer to unplug both ends, supply 12V to one end and see if the other end will illuminate an old sealed beam headlamp. If the circuit is intact, and both the old and new IAC have no function with scanner commands, I’d condemn the ECM

 

[HALLETT BOY]

Check circuit integrity back to the ECM. I prefer to unplug both ends, supply 12V to one end and see if the other end will illuminate an old sealed beam headlamp. If the circuit is intact, and both the old and new IAC have no function with scanner commands, I’d condemn the ECM

The problem with that is I can’t physically reach the ECU , the header and supercharger are in my way , and I’m not that flexible anymore .

 

[DaveH]

re-read post 8.

just beacuse you replaced the IAC doesn't mean jack if the new one has a different pin out.

furthermore if you can't get about 50ohm across each pair of your original IAC this PROVES the IAC is bad.

 

[C08H18]

I had a failed IAC 2 months ago. The behavior seems like a stuck IAC (in closed position) or plugged passage. IAC has two coils: one coil (energized by one pair of wires) moves the shaft in. Another coil (energized by a second pair of wires) moves the shaft out. The IAC does not report back the exact position to the controller. The controller runs a routine to find the closed or zero position. If you can't get a readout, it's more likely the controller. Good luck and let us know what you find.

 

[guest hs]

Can you see on the scanner what count it parks it self at? If memory serves me right it should be 99 counts

 

[BDMar]

re-read post 8.

just beacuse you replaced the IAC doesn't mean jack if the new one has a different pin out.

furthermore if you can't get about 50ohm across each pair of your original IAC this PROVES the IAC is bad.

This

 

[HALLETT BOY]

Update : probed all 4 pins of IAC connector , pins 1 & 3 had 12 volts , good . Pins 2 & 4 provide ground through computer to both coils in stepper motor . Found pintle all the way closed and motor stuck . Found throttle plate adjusted too far open , reset throttle blade to correct opening and put everything back together , all working properly now . Scan tool set for 800 rpm , engine now idling at about 775 . Engine seems to start quicker when cold also . Next , test on lake .