Occasional high idle and rough running/miss fire

[lakewaterjunky]

My 02 Sunsetter with the Monsoon has an unpredictable high idle condition that has me perplexed. This bothersome condition started at the end of last season.

After some research, (this site and the shop manual) It seems that the MPFI temp sensor has been known to be a culprit for this and possibly similar conditions, but it is reading properly across the board when tested (resistance check from cold through hot condition). The numbers were right on the money all through the temperature range listed in the Indmar shop manual table chart for this sensor.

The fuel filter is absolutely free flowing, the plugs look great and there is no visible reason that I can find that is causing this condition. The fuel pump has not changed its quiet operation, all the electrical connections look good and there does not seem to be any bad vacuum lines or leaks.

The boat always fires right up and runs every time, but the high idle comes and goes randomly. When it happens, it will run down the lake, pull a skier, or anything just fine only when it acts up, it seems to have a little miss every now and then at speed.

I have even readjusted the PP linkage so that it really pushes hard against the butterfly arm at idle so there is no way in the world it is coming off the set screw when its in the closed position. I have taken the air filter top off during this occurrence to see if it is in any other position besides absolute idle and every time it is bottomed out hard against the idle set point.

I do all of my own work on anything we have and it would be VERY tough for me to actually pay someone an exorbitant fee to diagnose this when I feel that it is just under my nose and it is being blatantly overlooked.

In fact, I’d much rather have to buy an analyzer/code reader for the few times it would be needed (this may be the first btw) than lay down the cash I could use for parts, only to have someone say ”there’s your problem, now that’ll be a hundred dollars”!!!

Has anyone else suffered this condition or circumstance? Info please.

[electricjohn]

Only one thing can cause high idle on a F.I. engine. Excessive air getting into the motor. The MAP sensor sees the drop off in vacuum and the ECM compensates with more fuel. So either you have a vacuum leak or your IAC (idle air control) motor on the throttle body is loose or acting up. The IAC allows air to get around the throttle plate thru a bridge port built into the throttle body. I would first check all and any vacuum hoses, then make sure everything on the intake manifold, plenum, and throttle body are tight.

[99response]

An IAC would have been my guess. Usually they only mess around with low RPM/idle but can also cause weird stuff to happen at higher RPM's if they get stuck open. Never seen that on a Monsoon though.

[lakewaterjunky]

Thank you for your replies electricjohn & 99response,

I’ll look farther into those suggestions.

I appreciate the input

[68Slalom]

I second a possible vacuum leak, you can check it first with propane around the intake system. A friend of mine had a problem with his boat and 2 shops tried to fix the problem all stating that it was PC related or MAF sensor, we found a small vacuum leak on the spacer (cracked) between the throttle body and intake manifold.

[footnlongline]

I'd take a hard look at the IAC as well, good luck

[lakewaterjunky]

Just to follow up on this in case someone else has the same circumstance, my boats condition turned out to be the TPS (throttle position sensor). The test is very similar to the MAP sensor check, instead of using a variable vacuum with a supply reference voltage (5-6VDC), you are just changing the feedback voltage by rotating the potentiometer inside, as if it were being turned by the butterfly/throttle shaft opening.

The MAP sensor and the AIC were both removed, cleaned & tested before diagnosing the TPS.

Autozone and other auto parts stores have some good technical specs and testing procedures listed online for several type of sensors and devices in case you don’t have a good shop manual or access to such information.

I could not directly cross reference the AC part #s that were listed on the bad TPS, not even through the AC Delco website. Even after calling the customer service phone #, they were unable to find it.

So I checked with a few auto parts suppliers online and locally, using the reference of a 1996-1999 GMC/Chevrolet 5.7L MPFI.

Even though mine is a 2002, evidently 99 was the last year for the most part, that the 5.7L was used widely, (before the displacement change in auto/truck production).

Seeing what the GM/Indmar TPS for my year Monsoon engine was suppose to read resistance wise, and comparing that with the sensors found, I was able to find the exact sensor for $36.00 with a two year warranty in stock locally.

Our Sunsetter came back to life and is running awesome, in gear she idles through the no wake zone @ just a little over 600 rpm. It’s really great to keep from going in and out of gear to slow down.

Even though the TPS wasn’t suggested, I really appreciate the responses from everyone that posted a reply trying to help. Thank you all for offering your input.

I hope this resolution will assist someone else in a similar situation.

J. H. Davidson

[68Slalom]

Just to follow up on this in case someone else has the same circumstance, my boats condition turned out to be the TPS (throttle position sensor). The test is very similar to the MAP sensor check, instead of using a variable vacuum with a supply reference voltage (5-6VDC), you are just changing the feedback voltage by rotating the potentiometer inside, as if it were being turned by the butterfly/throttle shaft opening.

The MAP sensor and the AIC were both removed, cleaned & tested before diagnosing the TPS.

Autozone and other auto parts stores have some good technical specs and testing procedures listed online for several type of sensors and devices in case you don’t have a good shop manual or access to such information.

I could not directly cross reference the AC part #s that were listed on the bad TPS, not even through the AC Delco website. Even after calling the customer service phone #, they were unable to find it.

So I checked with a few auto parts suppliers online and locally, using the reference of a 1996-1999 GMC/Chevrolet 5.7L MPFI.

Even though mine is a 2002, evidently 99 was the last year for the most part, that the 5.7L was used widely, (before the displacement change in auto/truck production).

Seeing what the GM/Indmar TPS for my year Monsoon engine was suppose to read resistance wise, and comparing that with the sensors found, I was able to find the exact sensor for $36.00 with a two year warranty in stock locally.

Our Sunsetter came back to life and is running awesome, in gear she idles through the no wake zone @ just a little over 600 rpm. It’s really great to keep from going in and out of gear to slow down.

Even though the TPS wasn’t suggested, I really appreciate the responses from everyone that posted a reply trying to help. Thank you all for offering your input.

I hope this resolution will assist someone else in a similar situation.

J. H. Davidson

Good to hear you got it,

Thanks for the update I was wondering if you would post up.

[JustinOSU]

What TPS (brand/model) did you purchase locally? I am having similar issues and would like to replace mine.

[SFD]

Chevythunder.com has some good articles & specs for fuel injection - all types that have been produced.

[lakewaterjunky]

What TPS (brand/model) did you purchase locally? I am having similar issues and would like to replace mine.

I just tried to get the part # and the link for the Autozone page it is listed on, but it says their server is down right now.

It is a Duralast TPS, if you use a 1999 Chevy/GM Suburban with a 5.7L MPFI engine it will come up.

Advanced auto parts had one that looked very similar and even had the same readings & body, but it had an assisted return spring built into it, the one at Autozone was a perfect match.

If their server comes back later, I’ll try to get the link posted or send a PM.

Good luck

[lakewaterjunky]

Here is the one I used TPS sensor

[electricjohn]

Thanks for the update on Aug. 18. I am really surprised you did not get a code 21 or 22 from your ECM. Evidentally it takes a total or near toal failure of the part to throw either of these codes. I would have never suspected a TPS problem would cause idle problems. Your update enlightened me.