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Mercruiser 5.0 EFI 2001 with Idle problems

D

DennisK

New member
Been dealing with idle issues for second season now and still trying to resolve it. I have a high idle and if you put the throttle against the idle screw it stalls out. Have replaced parts, TPS,Map and IAC and also switched ECM since I have Twins and no luck. Also have checked readings with scanner and IAC is at 0 and TPS is at 7.4 at a high idle of around 900. Looked for Vacuum leak but no luck. Above idle runs great. DennisK
 
Sounds like air maybe, I had similar issue and replaced the gasket on the throttle body and works fine now. They are cheap "cardboard" gaskets.
 
I'd start with making sure the throttle cable is adjusted correctly...and then start looking for a small vacuum leak...probably faster just to change all of the vacuum hoses as they do have some age on them. probably wouldn't hurt to make sure the throttle plate is clean as well as the IAC ports.

You can also put the scanner on the other engine and note the RPM, the MAP, the TPS and the IAC values once the engine is warmed up...at a normal Idle, the TPS value is usually 0 and the IAC value is in the range 35-60 (give or take).
 
makomark said:
I'd start with making sure the throttle cable is adjusted correctly...and then start looking for a small vacuum leak...probably faster just to change all of the vacuum hoses as they do have some age on them. probably wouldn't hurt to make sure the throttle plate is clean as well as the IAC ports.

You can also put the scanner on the other engine and note the RPM, the MAP, the TPS and the IAC values once the engine is warmed up...at a normal Idle, the TPS value is usually 0 and the IAC value is in the range 35-60 (give or take).
Click to expand...
Thks for your reply. I am going to change that throttle body gasket and hoses and look for any small leaks. Strange thing is with the scanner the TPS value was .74 and IAC was at 0 with an RPM of about 900. Also going to adjust that throttle against the stop screw but when I did that it would stall. I will also look to do a IAC and TPS relearn to get the TPS to 0 and IAC around .40-.50. This is my second season with this, cannot seem to get it right. Compression good, plug and ignition good. Scan values of other engine which is fine is TPS .50 , IAC 14 , MAP 2.11 , Idle at 660, Spark Adv of 5 degrees and hours of 328. Dennis
 
The IAC has nothing to do with idle speed. It is an air bypass that keeps the engine from stalling when you decelerate.
 
The IAC has a key role in maintaining the idle speed...

Maybe worth getting a copy of the MEFI3 MANUAL if you don't already have one...
 
There are basically two inputs on your EFI. Speed, determined by the spark pulse width (RPM), and density, determined by the MAP sensor. Everything else simply fine tunes these two. Your IAC is an output from the ECM. That is, the ECM doesn't "read" the IAC signal, it sends voltage to the IAC to tell it how open or closed it should be. If the swapped ECM produces the same results, you can eliminate the IAC from the mix.
 
The throttle cable should be adjusted so the throttle plate is closed and don't adjust the stop screw.

If the engine won't idle after the adjustment, without opening the throttle, then there is something not correct in the IAC subsystem...its possible the IAC that was replaced is still fine and that the issue is with the connectors or wiring...

If the TPS shows the throttle is opened more than ~ 5%, then the IAC won't be adjusted by the ECU...
 
Thks for your info. on my challenging issue. It is interesting what you are saying about my TPS which is reading 7.4 and I guess this is why my IAC reading is 0. I will first try to get the throttle against the stop screw which hopefully lower that 7.4 reading and also open up the IAC reading from 0. Will have to do a TPS and IAC reset. Connectors, wiring and possible vacuum leak will not be that easy to find. D
 
I'd suspect the throttle cable could just be removed and the spring should hold the throttle plate closed....at which point you could check the TPS with the scan tool before starting the engine...and most of the scan tools will provide multiple looks at a given parameter; ie TPS may show PERCENT OPEN or it may show VOLTS...
 
Yes you are right , my reading is in volts, starboard or good engine is .50 Volts and Port engine with idle issue is .74 Volts at an idle of 900RPM. Maybe getting that throttle against the stop screw will get the TPS down to lower volts with lower rpm. Right now if I did that the engine would stall but my hope is a reset with TPS and IAC with engine off might be answer with IAC opening up from 0 with TPS having a lower reading. The funny thing about this is the Port engine with idle issue runs great above idle. I just hope that I am not dealing with a vacuum leak somewhere. Thks
 
The scan tool may also have a THROTTLE CLOSED < YES or NO> display...if the ECU think the throttle is open, it won't attempt to regulate the IAC....
 
I did not see that option on the scanner so should not be an issue. The other thing I am not sure about is setting up the idle. Some Techs are saying have the throttle blades against the throttle body and let the IAC handle it from there while others are saying adjust the throttle blades just open enough for enough air to idle. Keep in mind this is EFI and have the TPS to calibrate. Thks, D
 
i don't think there is any 'calibration' required regarding the TPS...

as far as the Idle stop screw, I usually don't mess with them as they only set the minimum airflow at idle. If it needs to be reset, I'd start with a TPS value between 0.4 and 0.5VDC...

I'd still suggest making sure the throttle cable isn't a contributor to your 'issue'... just disconnect it from the throttle shaft pin...

BTW, "EFI" is a big vague - TBI or MPI is a much better decsriptor.

and for completeness, here's the quote from the MCM service manual re IAC : "The purpose of the IAC valve assembly is to control engine idle speed, while preventing stalls due to changes in engine load."
 
When I would unhook the cable and lower the idle against screw it would stall but run OK with a high idle of 8 to 900. I guess a possible vacuum leak? Seems to be 2 opinions about throttle blades, one is for the blades to be shut and the other is to be open slightly so it can idle. Mine is shut with a IAC reading of 0 and TPS reading of .74. Maybe I should open throttle blades with idle screw and see if it runs at idle with slightly more air. Is my problem too much air or not enough? Thks D
 
if your Throttle Plates are CLOSED (against the stop screw) and your TPS reads 0.74VDC and your idle is 800-900 RPM with the IAC at 0, I'd think either 1) there's a small vacuum leak or 2) somebody has turned in the stop screw...you can pull the throttle body and inspect the throttle plates to see if they really are closed. you can compare it to the TBI unit on the other engine.

another option is to see what the TPS reads with the KEY=ON and the engine not running...if its stays as 0.74VDC, I'd be inclined to back the idle stop screw out until it more like 0.5VDC...too much air flowing thru the throttle plates will keep the idle speed high and the IAC likely won't be activated by the ECU.

When the system is setup correctly, the throttle plates should allow enough airflow to maintain a minimal idle - usually somewhere just under 600RPM and the ECU drives the IAC valve to maintain the programmed idle speed (usually about 650 RPM). The MEFI-3 units have a load compensator tweak to the software but that only comes into play when you shift into gear...

In the mean time, it will pay you BIG dividends to get a copy of the MEFI-3 manual off of the www and then read and understand the sections covering idle controls...
 
EFI (MPI) is a bit out of my wheelhouse but i do remember that if the throttle body is dirty, especially where the butterfly makes contact when fully closed it can cause idle problems. Cleaned real well and resolved issue.

Again, I claim no expertise, just throwing it out there.
 
ok, , 1st off, is this an MPI, or throttle body engine?

the AIC absolutely controls idles speed as another poster stated, " "The purpose of the IAC valve assembly is to control engine idle speed, while preventing stalls due to changes in engine load."

disconnect the throttle cable and let the plate close, what is the reading on the scan tool for the tps sensor? even if the engine stalls, you can read the values. compare your readings to the other engine.,

1) are they the same w./ KOEO? (key on engine off)
2) if the values differ much, with the engine off, open the throttle body butterfly and take a photo of the inner area, walls and plate. is that area dirty?
as kghost said, clean the throttlebody , clean the walls, the plate, top and bottom, and replace the muffler (small mesh block)
recheck the TPS value, did it get more in line with the good running engine?

next, if there was a vacuum leak it would cause the idle to be higher cause the idle speed is controlled by the amount of air coming in,. since it wont idle with the plate closed, there is obviously not enough air getting by the plate.

when the throttle linkage sits against the set screw (non adjustable on MPI engine) does the engine idle drop below 600RMPS then stall?

respond the this post and i will direct you on where to go from here.

oh, btw, the fact it runs fine off idle indicates the problem is in the idle circuit,

one other thing you can do is swap all the parts from the good engine to the other.
is the IAC a mecruiser part oo aftermarket?>
i have had a ton of problems with the aftermaket ones, even the ones from ford. ( this is a ford part repurposed for merc)
 
Thks alot for your info. and I think we are on track in solving this . By the way it is Twin Mercruiser 5.0 EFIs 2001. I have swapped parts from the other engine and have also bought a new Quicksilver IAC and TPS and have taken the throttle body off and cleaned it and replaced with new gasket. The readings on the engine with the high idle are TPS at .74 and IAC at 0 with idle of 900RPM and the other engine is TPS at .50 and IAC at 14 with idle of 660. My hope is to set the throttle against the screw even if it stalls and and see if the IAC could take over from there and control the idle. The other option is to open those throttle blades slightly and see if that works. Have winterized both engines so will hit this again in the spring. Your thought? Thks again. D
 
when you say EFI, is it throttle body or multi port fuel injection?
when you get back into the boat, install those new parts and take readings at idle( throttle cable disconnected. then take readings from the other engine.
 
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