Fast Idle Issue

[Geronimo36]

I'm not sure, I don't have the diagnostic manual to see exactly. Maybe someone else knows?

Also it might be good to check the seat of the IAC cavity for carbon build-up. Also, maybe your new IAC is messed up as well?

Someone had a simalar problem on here once before and they replaced the TPS, still the same problem. Then swapped IAC's from one engine to the other and it turned out the new IAC was bad...

[Warlock28SXT]

I'm not sure, I don't have the diagnostic manual to see exactly. Maybe someone else knows?

Also it might be good to check the seat of the IAC cavity for carbon build-up. Also, maybe your new IAC is messed up as well?

Someone had a simalar problem on here once before and they replaced the TPS, still the same problem. Then swapped IAC's from one engine to the other and it turned out the new IAC was bad...

I cleaned the seat & cavity. I never thought it could be a failed new one, but I agree, I'll look into it. Thanks for your input.

[Ratickle]

ANy new updates? I'm feelin your pain and am ready to start PM'ing people.....

[Warlock28SXT]

ANy new updates? I'm feelin your pain and am ready to start PM'ing people.....

Nothing other than Geronimo36's wich I have looked at. He did bring up a good point about another bad IAC. We shall see. Thanks
p.s. No need to PM anyone on my account. It is what it is.
Thanks to MERPerformance and Geronimo 36 for the help/guidance

[Geronimo36]

Warlock, I called my buddy who was my boss/mentor when I worked at the marina... He had a good suggestion... When it's idling high, check the timing because the TPS controls the timing curve. That's somehting I had not thought of. He's also going to look up the TPS output readings for us.

Another suggestion, check the throttle plates shaft/bushings for wear. That's a GM TBI system from the 80's and they were known for wearing out the bushings and causing an air leak.. It might not be the problem but it's just a suggestion...

Have you also checked the Map sensor readings? If you have an air leak it will show up.

[Warlock28SXT]

Warlock, I called my buddy who was my boss/mentor when I worked at the marina... He had a good suggestion... When it's idling high, check the timing because the TPS controls the timing curve. That's somehting I had not thought of. He's also going to look up the TPS output readings for us.

Another suggestion, check the throttle plates shaft/bushings for wear. That's a GM TBI system from the 80's and they were known for wearing out the bushings and causing an air leak.. It might not be the problem but it's just a suggestion...

Have you also checked the Map sensor readings? If you have an air leak it will show up.

Thanks Geronimo36, I will look into these things as well. THis is exactly the kind of info I am looking for, what all ties in/plays a part. I will have the boat out tomorrow and I take a look. Thanks again.Jason

[MERPerformance]

Sorry still not fixed yet, been working 14 hr days. The ecm has a set idle rpm, could it be shorting out or getting a voltage reading that changes the IAC? I'am out of ideas to help, Maybe you can call Mark @ Precision Marine.I would check for air leaks first. Any chance the intake is leaking at the port gasket

[Warlock28SXT]

Sorry still not fixed yet, been working 14 hr days. The ecm has a set idle rpm, could it be shorting out or getting a voltage reading that changes the IAC? I'am out of ideas to help, Maybe you can call Mark @ Precision Marine.I would check for air leaks first. Any chance the intake is leaking at the port gasket

Thanks, I totally understand. I will be checking more out today on the lake. I have yet to monitor the engine while this is happening. I will let you guys know what I see later tonight. I am going to bring the old IAC with and maybe swap it out as well. Thanks again. Jason

[Ratickle]

Thanks, I totally understand. I will be checking more out today on the lake. I have yet to monitor the engine while this is happening. I will let you guys know what I see later tonight. I am going to bring the old IAC with and maybe swap it out as well. Thanks again. Jason

Any updates?????

[Warlock28SXT]

Any updates?????

Kind of..... I ran the boat laptop hooked up slowed to idle took it out of gear and it stayed @ idle. Huh? Ran it a couple more times and it stay @ idle. So I was now a bit confused. I had the boat docked (we were @ a party on the lake) I had to move the boat for some people. Got it fired up moved it gave it a little gas backing up back to idle out of gear and then it did it(fast idle again) you know I did not have the laptop hooked up just to move the boat.
Took it out again seeing as it now happend and agian when I was monitoring it would not do it. Packed away the laptop and went to take the boat out. All was still working normal. When I drove it on the trailer and came back to idle and out of gear it did not just fast idle it was "hunting" RPM's 700-1200 up and down up and down. Did not get it out Yesterday. Jason

[Geronimo36]

How much vacuum is the engine pulling at idle? Could also be a vacuum leak and when the engine is cold vs. hot it causes it to leak, eg intake gasket.

Personally I'd try and get a new IAC that you could put in and verify the new IAC you bought is/was bad but I guess that's hard not having parts available for testing... I hate changing out parts but sometimes when it's an intermittent problem it's good to start fresh again.

It could also be a voltage problem with the ECM. May also check all grounds to the ECM.

I hope this isn't too much info but I'd probably start from scratch and just take a systematic approach. It seems like it's going to take some time to diagnose since it's intermittent and kinda makes it harder that you have a single engine.

I just had a weird intermittent problem last week with my buddy's boat. We put a new 357 Magnum in it and above 4k rpms it would intermittently pop out the carb... Since the engine was brand new and sat for years I thought maybe something was stuck in the carb... Sometimes it would happen and other times it wouldn't.. On a huntch I decided to recommend replacing the ignition coil and now the boat runs perfectly... He also picked up 10 mph with the new engine over the old!!!

[Geronimo36]

Warlock, I got the TPS info for you.

The system is a 5/5 system meaning it uses 5 volts input for reference to run the system. So from the ECM you should see 5 volts on the input wire. From there you read voltage on the output wire (3 wire system, input, output, ground). The TPS voltage reading at idle should be .5 volts, WOT should be "near" 4.5 volts. There are no steps in-between and you should see a smooth voltage transition as you open the throttle from idle to WOT.

Hope this helps!

[rvw]

In addition to the voltage requirements for TPS as previously mentioned make sure the percent of TP is 0% when the throttle is closed. If the % of TP is above a certain value the ECU will think it is of idle and the idle control program will not be running.

One more thing you can look at with MEFI is the the Desired IAC position along with the Actual IAC position. Same is true for the Desired Idle RPM and Actual RPM. If the Desired is close to the Actual it is not your IAC.

Rick

[Warlock28SXT]

Sorry guys it been a while. I've been trying to put some cash in my pocket. The boat is hooked up to the truck and I'm heading to the lake shortly.

[Warlock28SXT]

Warlock, I got the TPS info for you.

The system is a 5/5 system meaning it uses 5 volts input for reference to run the system. So from the ECM you should see 5 volts on the input wire. From there you read voltage on the output wire (3 wire system, input, output, ground). The TPS voltage reading at idle should be .5 volts, WOT should be "near" 4.5 volts. There are no steps in-between and you should see a smooth voltage transition as you open the throttle from idle to WOT.

Hope this helps!

Geronimo Thanks. I was looking for the step count. Is it 0-100? I appreciate the help. Thanks again Jason

[Warlock28SXT]

In addition to the voltage requirements for TPS as previously mentioned make sure the percent of TP is 0% when the throttle is closed. If the % of TP is above a certain value the ECU will think it is of idle and the idle control program will not be running.

One more thing you can look at with MEFI is the the Desired IAC position along with the Actual IAC position. Same is true for the Desired Idle RPM and Actual RPM. If the Desired is close to the Actual it is not your IAC.

Rick

Rick If the desired vs act are close then what? How close is close? Thanks Jason

[Geronimo36]

Geronimo Thanks. I was looking for the step count. Is it 0-100? I appreciate the help. Thanks again Jason

Maybe I misunderstood your question, I'd have to look again.

Diagnostic manual didn't talk about step-count from what I read, only voltage reading from idle to WOT. The TPS voltage reading at idle should be 0.5 volts, WOT should be "near" 4.5 volts. Should see a steady increase in voltage from idle to WOT.

[Warlock28SXT]

Maybe I misunderstood your question, I'd have to look again.

Diagnostic manual didn't talk about step-count from what I read, only voltage reading from idle to WOT. The TPS voltage reading at idle should be 0.5 volts, WOT should be "near" 4.5 volts. Should see a steady increase in voltage from idle to WOT.

I'm heading out now, when I record the info it gets it all TPS voltage and step count. I was just wondering, the voltage readings are listed in my manual. I hope to find something. Thanks again Jason

[Geronimo36]

No problem, good luck!!!

[Warlock28SXT]

No problem, good luck!!!

Well it started idled fast and then ate the raw water pump impeller. I will get one asap and try again. TO BE CONTINUED.....................