1997 Merc 5.7LX Knock Sensor tshooting

[Mike220]

I have a 1997 Merc 5.7LX with Tbolt V and the knock sensor. I have been chasing a WOT issue since I have had the boat (6 years). The boat basically will hit 4000-4300 rpm's regardless of prop pitch. I am running a 17 pitch now. This is a 22 ft. bowrider.

I have always suspected timing. The boat has decent acceleration and will pull a skier, but I think there is more to be had.

So I tested the Knock module/sensor by hooking my dvm to the purple wire and ground. I believe it is supposed to deliver 8-10 volts. I get 4.5 at idle and it jumps around. At 2000 rpm, it sometimes hits 8v, but most of the time it is under that.

I checked the timing and at 2000rpm's I am getting 24d of total advance. Looking at the Merc SB's this should be up around 35-40d (if I am reading correctly). It seems like the knock module is retarding the timing. This is also the case if I unplug the knock sensor.

Before I replace the knock module, I am curious if it could be something else. I would think the module would work or not - the erratic behaviour is confusing.

 

[dubs283]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

to test the knock module:

1- set rpm at 1500 or so in nuetral

2 -rap sharply against engine block with a hammer next to knock sensor

3- note any change in rpm, if the rpm's drop/fluctuate, the knock sensor/module is operational

2000 RPM will not fully advance timing

as far as timing, do you know how to properly check/set the t-bolt V ignition timing??

when was the last time you had a FULL tune-up??

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

Distributor cap, plugs, and wires are about 3 years old with maybe 30 hours on them. I've been checking timing by grounding that purple wire, starting the motor and hitting it with a timing light. At idle it is 10d. When I rev the engine I seem to get more advance in "base" timing mode then I do without the purple wire grounded - although I need more scientific testing to be sure.

I tried the knocking technique and nothing changed. I swear I read somewhere that to test the actual knock module you measure voltage at the purple wire and it should be 8-10v. But I can't find that service bulletin anymore.

I also am a little confused reading the Merc timing specs. It appears that by 2000 rpm I should have a minimum of 35d advance if the knock sensor isn't retarding it.

 

[dubs283]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

you do have the correct OEM manual for your engine??

how are you finding service bulletins??

keep in mind, mercruiser does not include base timing when displayed on a curve, one has to add their base timing reading, in your case 10 degrees, to the timing/RPM curve in the manual

as far as a voltage/ohm reading for the module, i have no idea, i just know that when you bang by the SENSOR, you should note an RPM change at about 1500-2000 RPM

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

Update. I opened my Seloc manual and found the testing procedure for the Knock Module. Basically you start the engine and measure voltage at the purple/white timing lead. It should read 8-10 volts. Mine reads 4.5 volts and fluctuates all over the place. I started tracing wiring and found the ground wire that comes out of the ICM and grounds on the ICM mount was loose. I tightened that and in the process realized I DO need a new cap and rotor. The moral of that story is always check never assume.

Once I get the cap and rotor, I'll do more testing today with that ground wire tight. I wish this was a ford engine with the distributor up front....it is hard getting to the back of the engine.

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

Update. After tightening the ground and replacing dist cap/rotor I am still only getting 4.5 volts at the timing lead (should be 8-10). The manual leads you to believe you either get voltage or you don't. My theory is that the knock module is bad and is not letting the TB5 advance the timing to full specs. It seems I am about 10d less than I should be a various rpm's. Idle is fine.

I am ordering a new knock module and hopefully that fixes it.

 

[dubs283]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

I opened my Seloc manual and found the testing procedure for the Knock Module

thats one problem right there

get rid of the seloc manual and use an OEM manual for your engine

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

Update. I received the new knock sensor and downloaded the Merc manual. In this situation Seloc must of cut and pasted from the Merc manual because they read exactly the same.

Anyway, installed the new knock sensor and still am reading very low voltage at the timing lead. My next step is to troubleshoot the circuit (I should have done this first). According to the Merc manual the pur/white circuit starts at the knock sensor -> goes into the wiring harness where it tees off to the timing lead -> then goes into the ICM. I unplugged the knock sensor and on a whim measured voltage on the pur/white terminal. It read 2.5v. I would think it should have read zero since the knock sensor provides the voltage to this circuit.

I am not very experienced in troubleshooting circuits/electrical. Does it sound like I could have a short or open in this circuit? Most of this circuit is encased in the wiring harness which is taped tight as a drum. Should I dig into that? I am wondering if where it tees to the timing lead could be an issue.

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

I am the only one replying to my own thread

I think I found my problem. I took of the ICM and guess what - it sounds full of water.

I've read a little about this problem but really thought it would put it in a "no work" condition. My just would not advance the timing.

Are these things recoverable?

 

[maniogre]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

You know more than I do but keep up the good posting. I'm interested as I have the same engine and maybe the same problem.

Thanks,
ogre

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

I dried the ICM out over night and will try it this afternoon. I have no confidence it will work.

It is very strange because the unit seemed like it was sealed well, but by shaking it you could hear the water. And if I squeezed, water came out. So I opened it up. The internals are covered in a gooey substance, I suspect to prevent moisture damage. But having an ounce or two of water IN the unit cannot be good.

By searching the internet it seems the 1996/97 5.7LX motors/ICM's are prone to this issue. Mercury has superceded the ICM with a part that looks completely different - but it costs about $450.

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

I let the ICM dry overnight. I slathered the back of it with Thermal grease and put it back in. Motor started but I still had did not get the timing advance indicated. At 2000 rpms I am only getting 18 degrees total advance.

I ordered a new ICU today. I found an older post on this site that had almost the exact same problem. The Mercruiser manuals only have troubleshooting steps for a No Spark situation.

I'll post back when the new ICU comes in. FYI - the new part looks completely different then the old part.

 

[dubs283]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

///

 

[Mike220]

Re: 1997 Merc 5.7LX Knock Sensor tshooting

Final update. I installed the new ICU. It is a new part # and looks completely different. You have to splice it into the wiring harness as it has two plugs instead of one. All in all - not too bad of an install, the instructions were very good.

And best of all - it fixed my problem. I am getting full timing advance and the boat performs significantly better in both acceleration, cruising and WOT.