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View Full Version : LT-1 Vett engine wont idle, somtimes.


dudeman
06-23-2005, 11:15 AM
Just found this forum, lots of good info already. I searched previous threads with no exact similarity of problems with my combo. Any way the boat is a 99 Maristar with the 340 hp corvette engine. Has ran perfectly since new until two weeks ago. Problem: After engine has been initially started and allowed to warm up, a quick 10 min drive to our favorite cove. Slow down to no wake click into neutral and the engine dies. This problem has been intermittant within two weeks. I changed the new plugs, champions with the previous years autolites and it ran like a champ, that day. Next day the engine wont idle. No other modifications have been done yet other than filling up with gas. The engine only has 200 hours on it. The only way to get the engine to run is throttle up quickly while cranking. The engine operates normally above idle with no power loss, or excessive heat etc. This engine operates at 160 degrees. Obviously this is a dangerous situation and very frustrating. Any ideas would be appreciated.

sizzler
06-23-2005, 11:51 AM
welcome to the board........

not my forte ,,but i'm sure engine nut and the other mechanics can help,,ive replied to keep u on the home page.....

dudeman
06-24-2005, 12:24 AM
Well not many takers on this topic it appears, maybe its so simple a fix I should know it. For any interested I tried some seafoam injector cleaner hoping it would help. My wife started the boat and launched with our 3 yr. old. Upon returning to the water I was startled to see them floating helplessly. After swimming to the boat, I temporarily removed the battery terminal in hope of the computer resetting and adjusted the throttle linkage to achieve a higher idle. The boat finally ran after being swarmed by some maliboo hoos and a few jet skiers. I dont know exactly what worked but I ran it out for about 30 minutes. Slowed and selected neutral. Do you think it idled? Of course its a MC! Im going to purchase a factory service manual next week probably will be helpful, I wish I would have had one in my tool box. If anyone has had a similar experience or maybe some imput on a permanent fix it certainly would help my state of mind. Thanks for any interest or suggestions. By the way the beer elephant did in deed own the lake today.

bucky
06-24-2005, 01:06 AM
Don't get discouraged. You asked a pretty tough question involving either the fuel system or ignition, and only a couple people here (me not one) can give an answer without hands-on info. Hang in and good luck!

Andyg
06-24-2005, 01:35 AM
I had a similar problem with my 97 PS190 with the LT-1. The first two years I had it the engine would die sometimes when returing the throttle to neutral. I think the dealer had to clean the IAC. I don't know how to do it. IAC I believe stands for Idle Air Control. I think it automatically adjusts the amount of air it allows into the engine when you are in neutral.

Engine Nut
06-24-2005, 08:52 AM
Just found this forum, lots of good info already. I searched previous threads with no exact similarity of problems with my combo. Any way the boat is a 99 Maristar with the 340 hp corvette engine. Has ran perfectly since new until two weeks ago. Problem: After engine has been initially started and allowed to warm up, a quick 10 min drive to our favorite cove. Slow down to no wake click into neutral and the engine dies. This problem has been intermittant within two weeks. I changed the new plugs, champions with the previous years autolites and it ran like a champ, that day. Next day the engine wont idle. No other modifications have been done yet other than filling up with gas. The engine only has 200 hours on it. The only way to get the engine to run is throttle up quickly while cranking. The engine operates normally above idle with no power loss, or excessive heat etc. This engine operates at 160 degrees. Obviously this is a dangerous situation and very frustrating. Any ideas would be appreciated.

Sounds like you may have a sticking IAC (Idle Air Control Valve). I have included a photo to show the location of the IAC on the LT-1 throttle body. The IAC id in the upper right of the photo eith the yellow pencil pointing to it. Remove the two screws that hold the IAC in place and clean it up with some carb cleaner spray. With the IAC removed, reconnect the wires and hold the IAC between your thumb and forefinger (so the pintle does not come out). Have somone turn the ignition switch on. You should feel the pintle move out then back in. If it doesn't move, you will have to replace the IAC.

Engine Nut

Thrall
06-24-2005, 01:38 PM
I think engine nut's right, but you did say you had to throttle up while cranking over to get it to start. With this symptom, the throttle position sensor (TPS) could be malfunctioning as well.

dudeman
06-24-2005, 04:23 PM
Thanks for the encouragement everyone, very much appreciated. My engines problem certainly appears to be this "IAC". From Engine Nut's visuals and explanation this would control the air/fuel ratio at idle. This is where the engine stalls. It was and probably still is an intermittant problem which leads me to believe its sticking sometimes. Does the IAC take over interpreting info as the TPS reads the throttle is closed? My engine runs fine at 950-1000 rpm's and up. I will clean the IAC and check for pintle movement. Is there a measurement/voltage/resistance or anything that I can check against in a non running engine? I assume the pintle would not be stationary if the engine is running at idle with the IAC connected. I remember working on a throttle body which used a TPS which could be diagnosed or checked with a multimeter. Will the factory service manual provide such information? Thanks again for the insight!

Thrall
06-27-2005, 08:05 PM
Did ya' get it fixed?

dudeman
06-28-2005, 12:03 AM
Haven't got it fixed yet, trying to make time. Got to have it ready for the 4th! I will post back with details when its fixed probably Tue-Wed. A lot learned with a few minutes on line not bad. Thanks for the help I think many will benefit from my problem!

BG1772
06-28-2005, 02:11 PM
Dudeman, I have a feeling that my post was one of the ones you reviewed before submitting yours. I ended up having to replace both the IAC and the TPS to get my boat to run properly. Runs like a champ now. Not sure if the parts will be the same on your boat, but I found that my TPS and IAC are the same parts as for a '98 GMC Yukon. I was able to get the parts from NAPA and saved a lot of $$.

Kell
06-28-2005, 03:10 PM
Dudeman,

I also have a 99 Maristar 210 VRS, although with the LTR 330 MPI/EFI. This was a new boat to me last year, and I had a cold stall issue. When I would back the boat into the water, start it up, and throw it into reverse to back off the trailer, it would stall. When it was warm, it ran very well with no stall issues.

JimN and others helped me to trouble shoot this, and I beleive JimN told me that I should have the ECM recalibrated. So I took it to my local MC dealer, and they sent the ECM back to indmar, and it turns out that the ECM was 2-3 generations behind. It costs $50 to have it recalibrated to the most current generation, but I paid big time for express shipping and install by the dealer due to a family reunion planned for that weekend. Total bill if I remember correctly was about $100.

The recalibrated ECM completely cured my cold stall issue, and I immediately notice that the engine runs so much smoother at idle, so it was worth it to me. By you description, your issue seems to be unrelated to mine, but thought I would pass a long this info to you about the ECM.

Where were you able to get a engine mechanics manual, I would be interested in that as well. I have the engine operators manual, but that is pretty basic. Thanks.

Thrall
06-28-2005, 05:22 PM
BG1772/Dudeman,
The IAC and TPS may be interchangeable w/ auto parts. JimN would be able to verify.
Also, if you get a repair manual for the LT-1, let us know. I could use one myself.

dudeman
06-29-2005, 03:44 PM
Took off the IAC yesterday, it was very clean, no corrosion. I held the assembly as engine nut instructed. When the ignition was turned to the on position the pintle retracted inside the assembly 1/4 inch and stopped. This is not what I was expecting in accordance with Engine Nut's explanation. At this point I will assume the IAC is bad. When the ignition was turned off the pintle pushed/unscrewed itself out of the assembly. It appeared that the pintle could be adjusted by screwing the threaded end into the assemby. As to how much is correct, I have no idea. I turned the ignition on then off again while holding firmly against the pintle, it settled into a position close to original. Reinstalled the IAC. Started the engine and it idled fine. I suspect the original problem will happen again. If so I will replace the IAC. Thanks for everyones contributions, I will definitly check the auto parts guys before buying. I would guess that the only difference is probably aren't coated with marine paint on the metal surfaces. I can say I have a 94 LT-1 engine in an old Cruiser and the electrical connector for the IAC is only 3 wire instead of four. I think you guys are right as to using a 98 or 99 part instead, I will check. If my problem resurfaces after replacing the IAC and eventually the TPS, I will look at the ECM. I havent found a factory service manual yet. Thats probably what I should be focusing on.

jacklsl
06-10-2010, 11:40 PM
I have a 99 prostar with the lt1 340 hp engine, and exactly the same symptoms. I don't see and link to the image that shows where to find the IAC. And, I'm very curious to know what eventually cured the problem. My boat has about 600 hours, but ran perfect until a couple weeks ago. And it still runs strong if you can deal with the idle problem.

JLeuck64
06-11-2010, 12:47 AM
Ya... a LOT of really useful pictures were lost quite a few years ago.

Here's a pic of mine. I disconnected the harness from the idle air control valve so you can see it's location better. Driver's side of engine, below the throttle body.
HTH

BG1772
06-11-2010, 03:11 PM
On My 2001 X-Star I replaced both the IAC and the TPS (throttle position sensor). In the case of my boat, the TPS was the bad part. I found that the parts were available at NAPA if you tell them it is for a '98 GMC Yukon. It was the exact same part for a lot cheaper. I think it was around $22.

jacklsl
06-13-2010, 11:28 AM
Thanks for the picture JLeuck64, I found it. I pulled it out and cleaned it up as well as the air passage in the throttle body with some carb cleaner - and it is working again for the moment anyway.....

BG1772, your x-star motor is a LTR? I assume the 98 Yukon part and for your boat will not match for mine? I went to a few parts stores, and nobody seems to have any luck helping find it. I think I may be stuck spending for a marine item. The part numbers on the IACV are: 22193 C31168, but google doesn't seem to get me anywhere with those online, and they seem to help much at the parts stores.

BG1772
06-13-2010, 12:00 PM
Yes, I have the LTR motor. I am not that familiar with the difference between the LTR and LS1 motor. When I initally took the parts into the local NAPA they were no help matching the part to what I had. I just happened on an ebay auction that had a picture of an IAC that looked just like mine. The auction said it was off a 98 yukon. So I went to NAPA and bought an IAC for a 98 yukon and it was exactly the same. You could try taking the IAC with you to your auto parts store and asking them to pull one for a 98 Yukon to see if it is the same.

As for your cleaning the IAC, that is the first thing you need to do when the idle starts getting rough. if after cleaning the problem returns, it could be that the IAC is getting lazy and is not sealing properly.

The TPS is nothing more than a rheostat. if it moves really easily when the motor is off, it may be going bad.

Good luck!

JimN
06-13-2010, 12:45 PM
Yes, I have the LTR motor. I am not that familiar with the difference between the LTR and LS1 motor. When I initally took the parts into the local NAPA they were no help matching the part to what I had. I just happened on an ebay auction that had a picture of an IAC that looked just like mine. The auction said it was off a 98 yukon. So I went to NAPA and bought an IAC for a 98 yukon and it was exactly the same. You could try taking the IAC with you to your auto parts store and asking them to pull one for a 98 Yukon to see if it is the same.

As for your cleaning the IAC, that is the first thing you need to do when the idle starts getting rough. if after cleaning the problem returns, it could be that the IAC is getting lazy and is not sealing properly.

The TPS is nothing more than a rheostat. if it moves really easily when the motor is off, it may be going bad.

Good luck!

Moving easily isn't the issue- it's whether the potentiometer's return voltage is out of the normal range the ECM needs to see.

Also, in case the only one a parts store has is for an LT-1, that one is the reverse of the others. When the LT-1 TPS shows high voltage WRT throttle position, the others show low voltage. Other motors will think it's at WOT, which results in a 'clear flood' command and no fuel will be delivered.

jacklsl
06-13-2010, 01:49 PM
Thanks everyone for the help. It looks visually that the IAC for 96 Impala SS is the right part. I'll probably give it a try, and post the result to help anyone else with an IAC issue on an LT1.

Hoping I won't have to mess with the throttle position sensor, as I don't have any issues with anything other than maintaining my idle - it simply drops so low that the motor wants to quit. Once I'm in gear and controlling the throttle, it runs perfectly fine.

jacklsl
06-15-2010, 10:50 PM
OK, I found one that worked great. I have a 1999 Sammy Duval ProStar 190, with a 340 HP LT1. My IAC (Idle Air Control) valve stopped working, causing my boat to die on idle, or at least struggle to keep from quiting.

The IAC for a 1996 Camaro Z28 is the exact match. The part on my boat was stamped with: 22193 C3118, which is exactly the same stamp on the part I got from Advance Auto Parts. Their part is: #21965. I'm also fairly certain another part #307015 from AutoZone is also a match.

jasonhigham
07-09-2010, 08:39 PM
I think my TPS may be going bad and would like to get some thoughts. I have a 2007 X-Star with the MCX. After the engine is warm with the throttle in neutral the RPM's at idle will jump up and down about 300-500 rpm's. If I shut the engine down and restart it will fix the problem for a short while.

Any thoughts?

KHall
07-09-2010, 08:44 PM
Would look at the IAC first.

JimN
07-09-2010, 09:51 PM
I think my TPS may be going bad and would like to get some thoughts. I have a 2007 X-Star with the MCX. After the engine is warm with the throttle in neutral the RPM's at idle will jump up and down about 300-500 rpm's. If I shut the engine down and restart it will fix the problem for a short while.

Any thoughts?

If your TPS has problems, it will store a code. Scan it and find out before just throwing money at it.

BG1772
07-10-2010, 10:24 AM
I agree, I would start with the IAC. Might just need to be cleaned.

jasonhigham
07-10-2010, 11:34 AM
How do I clean the IAC?

Engine Nut
07-10-2010, 11:15 PM
I agree completely with JimN .. take it in and get it scanned. By the way, a 2007 model has throttle by wire. There is no IAC and the TPS functions differently than it does on a mechanical throttle body and is not a replacable part.The whole throttle body would need to be replaced to replace the TPS.

If your TPS has problems, it will store a code. Scan it and find out before just throwing money at it.