Mercruiser 5.7 LX Thunderbolt idle and timing problems

I have a 1997 5.7LX with the Thunderbolt ignition and AFB Carb, I think it is a D series as it has the pumps front and rear 4 barrel carb, which ran great some days and would not even start on others. On one of the days it would not start I found that I had power going into the ICM but none passing through (plus it somehow had water coming out of the ICM) so I replaced it. This is where the fun started. The original part was 806957-4 which even the factory had no record of such a part number so they insisted that the part was a 806957t4 which has been superseded (of course) to part number 807264A01 which is a conversion kit, so that is what I ordered. The old part said right on it that it was for the 5.7LX; however upon opening the new part the instructions (which were in the plastic which stated no returns if opened) stated that it was for all 5.7 within a certain serial number range (which my motor was in), but for engines with 2 barrel carbs. I called several dealers and the factory and they all insisted that this was the correct part, and that it was used for both 2 and 4 barrel motors as this is what both the 5.7 L (2 barrels) and 5.7LX (4 barrels) were superceded to. At this time I also replace the plugs, distributor cap, rotor, gasket, ignition sensor, and fuel filter / water separator right away. Now two problems:

#1) the motor is having a very hard time starting as it sounds like the timing is set too high, but it runs great once stated. If I back the timing off it starts easier but falls on its face. I have grounded out the purple and white wire to set the timing and adjust the carbs idle speed; however:

#2) the idle speed will not drop even when bypassing the module. It is up around 2000 RPM, and I cannot get it to do down. It will not respond to the idle screw. I have even removed the linkage to make sure that this was not the problem. The choke appears to be working properly and not hanging up.

Sooooo help! What next? Could this all be caused by the module which they insist is correct or am I doing something incorrectly? Of course there are no returns on electronic parts, even those costing $400.00, and which could be causing the problem. Is there some other way that I should be bypassing this new part? Please help, the season is so short up here and I do not want to miss another weekend.

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

What is your serial number? Do you have a Thunderbolt IV or V? Is it an Alpha or Bravo drive?

You also mentioned water was coming out of the ICM. Did you check the wiring harness going to the ICM for corrosion?

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Sounds like an older motor than 1997. The Q-jet was out of production a number of years before that. Weber 4brl carbs were on 1997. Also, the discription of water coming out of the ICM, sounds like the early box type module T4. 1997 had large silicone modules T5, that bolted to distributer. An LX would have had a ICM and knock module, back to back. Anyway provide the serial numbers and Don will tell you what you have.

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Don said:

What is your serial number? Do you have a Thunderbolt IV or V? Is it an Alpha or Bravo drive?

You also mentioned water was coming out of the ICM. Did you check the wiring harness going to the ICM for corrosion?

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The motor is a 1997 (confiremed by the factroy) # is oko14536, and it has an Alpha drive. I do not know what Thunderbolt it has but it has the knock sensor. I did not see any issues with the harnes itself.

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Re: Mercruiser 5.7 LX Thunderbolt idel and timing problems

Don said:

What is your serial number? Do you have a Thunderbolt IV or V? Is it an Alpha or Bravo drive?

You also mentioned water was coming out of the ICM. Did you check the wiring harness going to the ICM for corrosion?

Click to expand...

It was late, I was wrong the carb is a AFB Carter carb, I think it is a D series because it has the pumps front and rear.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

Just to confirm your procedure, the ignition switch needs to be OFF when you ground the purple/white wire.
Then disconnect the throttle cable from the carburator.
Start the engine and adjust the speed down to idle and set the timing to specs. Then adjust the carb mixture while at the proper idle speed.
Shut engine off. Disconnect jumper from ground to P/W wire.
Start engine and check that advance is working or not and report back.

PS: It's a Weber carb, not a an old Carter AFB.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

You do have a Thunderbolt V. The Thunderbolt IV did not have a knock sensor. I am curous to where you saw water dripping out of the ICM. Essentially, the modules are just bolted to a metal bracket with heatsink compound. Did the water drip out of the connector, after it was removed from ICM or just a little water ontop of the modules (splash shouldn't hurt it)? Unless there is corrosion in the connector, modules generally either work or don't.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

[quote time1153845014= userJohnS=_ uid44056= fid24= tid= rid6= You do have a Thunderbolt V. The Thunderbolt IV did not have a knock sensor. I am curous to where you saw water dripping out of the ICM. Essentially, the modules are just bolted to a metal bracket with heatsink compound. Did the water drip out of the connector, after it was removed from ICM or just a little water ontop of the modules (splash shouldnt' hurt it)? Unless there is corrosion in the connector, modules generally either work or dont'. [/quote]

The stuff that is supposed to seal it was actually damp, and shrunken so I was able to pop it out of the case. The water was dripping from within the unit itself. I let it dry in extreme heat for a few days but the damage was done, it would not work.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

Don said:

Just to confirm your procedure, the ignition switch needs to be OFF when you ground the purple/white wire.
Then disconnect the throttle cable from the carburator.
Start the engine and adjust the speed down to idle and set the timing to specs. Then adjust the carb mixture while at the proper idle speed.
Shut engine off. Disconnect jumper from ground to P/W wire.

Start engine and check that advance is working or not and report back.

PS: It's a Weber carb, not a an old Carter AFB

Same deal. The idle will not drop below 2000 and I have to drop the timing back to 5 to get it to start easily. I can get the timing to move, but when up at 10 it is a nightmare to start and will not start easily until down to 5.

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Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

Just replaced a 807264ao1 on a 97,5.7,alpha,gen2 yesterday.This beast would missfire,the tach would jump 3000 rpms and it would sometime backfire through the carb.Removed the T5 module from the back of the dist.and when I shook it,I could hear/feel water in it! I turned it upside-down and squeezed it and water came out of a place which had some white sealer over two unused terminals on the normal top of this part!!I I pryed off the cover and let it dry in the sun all day,still,bad part! I took the new part and re-sealed the entire cover with clear rtv before re-install.Runs fine now!

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

I/O said:

Just replaced a 807264ao1 on a 97,5.7,alpha,gen2 yesterday.This beast would missfire,the tach would jump 3000 rpms and it would sometime backfire through the carb.Removed the T5 module from the back of the dist.and when I shook it,I could hear/feel water in it! I turned it upside-down and squeezed it and water came out of a place which had some white sealer over two unused terminals on the normal top of this part!!I I pryed off the cover and let it dry in the sun all day,still,bad part! I took the new part and re-sealed the entire cover with clear rtv before re-install.Runs fine now!

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You state that you just replaced it...was the new part bad out of the box, or were you able to reuse the old one once you dried it out? My orginal part was dead due to water ( I tried to dry it out but it still would not work ), it is the new one that I'm having the problems with. Was the 5.7 you did this on a 2 barrel or 4 barrel? Did you notice on the instructions where it says that it is for 2 barrel motors? The factory insists that this part will work even though it is a 4 barrel, but I have tried everything I can think of and can not get this thing to run.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

The '97 5.7L alpha 2brls do not have a knock sensor, knock module, or wiring harness to support it. I would expect in base mode, it would ignore the signal from knock module, but you might try a quick test with it disconnected.

I have a 806956 ICM 5.7L/bravo lying around, and did inspect it closely. I see the panel you are talking about. :$

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

Don said:

Just to confirm your procedure, the ignition switch needs to be OFF when you ground the purple/white wire.
Then disconnect the throttle cable from the carburator.
Start the engine and adjust the speed down to idle and set the timing to specs. Then adjust the carb mixture while at the proper idle speed.
Shut engine off. Disconnect jumper from ground to P/W wire.
Start engine and check that advance is working or not and report back.

PS: It's a Weber carb, not a an old Carter AFB.

Click to expand...

The fuel pump is working fine and the carb appeared to be fine looking in from the top. As a last resort I took it off only to find that some knot head had trashed it. The plates were bent (one would close the other was open 1/8"), and that there was a bent shaft with a shear off screw head. The plates also had vice grip teeth marks on them, as if someone tried to straighten them out. I just ordered a rebuilt carb from Fly Fishing Carburetor in Largo FL ($228 plus $88 core...what a deal!) and should have it on Sat morn. The timing is working, but it is still hard to start which, I assume is do to the carb issues. I hope this is the end of the gremlin chase.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

You won't be able to adjust the base timing without getting the idle to specifications. If you have removed the throttle cable and backed the idle screw completely out of the throttle linkage, the throttle link arm should be resting on the throttle cam. If it is not, then I believe there is a return spring not doing its job or there is binding on the throttle plate shaft, or both. You should be able to visually see that the idle screw is not touching the throttle cam and that the throttle link arm is sitting directly on the cam when the idle screw is completely out.

Sounds as though you have the right ICM and that you have the base timing procedures firmly understood, but if the carb is forcing the idle speed, as it seems it is doing, then timing is a secondary issue. The primary issue appears to be a carb problem for the moment.

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

I think you've found your problem Duke...I was researching my post before I submitted it and you beat me to the punch. Good luck and hope you get on the water soon. Make sure you report back after you get the new carb on.

jb

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

Got it on the water and all is well. I have a 19 prop and was able to get about 47-48 mph on a gps. I wish I could get a bit more...the rpms were at a bit low ( about 4500 ) so I'm going to give a 17 a try (which I have but have not put on yet, both the current 19 and the 17 are both Hustler aluminum 3 blade props). I also have one of those fins on it so between that and an aluminum prop that may be all she has. The tach was spiking all the way up to 6000 but the motor did not sound any different when that happened, so I'll bet it is a tach and not a motor issue. If I hit the tach I could actuall get the needle to jump around. I can not get the warning buzzer to work at half key, so that is the next thing I need to look into. It still starts a bit hard but is starts with the help of a little lever without any real problem. All in all, I'm back on the water and that is all that matters for now!

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

It still starts a bit hard but is starts with the help of a little lever without any real problem.

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If you are trying to start a carbed engine like you do your car with an EFI engine, forget it.
With a carb you have to pump sometimes, back it off to about a quarter throttle and start it. May have to goose it (depending on how the choke is set) to keep it running till it warms up.
If you were expecting to just stick the key in, and start it at idle, you would be lucky to ever get it going.

Welcome to the world of Carbs! ! ! ! ! !


Ohhh, and what does this mean ? ? ? ? ? ? ?

I can not get the warning buzzer to work at half key

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Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

I think he is talking about the warning buzzer for oil, temp, etc that comes on when you turn the key to the on position, but do not start the engine. Much like the warning lights on the dash of a car.

Is the buzzer disconnected perhaps? I know the buzzer on my uncle's boat with a 5.7L/alpha is rediculously loud, but it sure forces you to get the motor started that much faster!

Re: Mercruiser 5.7 LX Thunderbolt idle and timing problems

Mine is loud too. I call it "the-prop-is-in-the-water" buzzer. It comes on at half key but goes away when the engine turns over.

Good for you for getting back on the water. The performance you are seeing is pretty darn good. Here's a resource on prop-ing your motor. Good luck and be safe.

http://www.caravelleboats.com/propeller info.htm