8.1 mercruiser keeps frying the computer

[Fun Times]

It' appear the early years of the 8.1's had a few camshaft sprocket designs while the crankshaft shows only one sprocket part number/design for all years.

Having your serial number and GM block engine code might help but being an 05, it should of been the newer design by then.

Some info of the new design for you.
http://www.bing.com/cr?h=c3i2yODrTJ...3E93E25BD5262A7049E38EABC5B6362&p=SERP,5140.1

Not sure if you've tried this or not yet, but since the tachometer is showing 3000 rpm at crank, you might want to disconnect the gray wire at the tachometer and see if it will start. A bad gauge will keep an engine from starting. Also it's a good idea to separate the boat wiring harness to the engine wiring harness and jump the engine harness to see if it will start.

 

[alldodge]

Hey FT if the gear can be put on backwards and not be lined up, can a visual inspection down the bore determine if this is an issue? Appears you should be able to remove the sensor and be able to see the gear and it should be in the center. Like all of us, just hate the idea of pulling the engine apart before all other methods have been tried

 

[Fun Times]

Hey FT if the gear can be put on backwards and not be lined up, can a visual inspection down the bore determine if this is an issue? Appears you should be able to remove the sensor and be able to see the gear and it should be in the center. Like all of us, just hate the idea of pulling the engine apart before all other methods have been tried

With removing both the crank and camshaft sensors, you may be able to use something like a digital inspection camera to help inspect the reluctance wheel and sprocket at both ends of the crankshaft.

At the front of the engine where the cam sensor is, you may be able to see if the cam and crank sprockets alignment marks line up.

The rear of the engine/crank has the crankshaft sensor/reluctance wheel (if it is on backwards). This site suggests that you put the stock wheel on their new crankshaft so my guess would be it is possible to be on wrong. http://www.mkhammer.com/new-scat-81-496-forged-crankshaft-p-302710.html

05 sanger, are you able to ask the builder if the reluctance wheel was removed?

http://www.harborfreight.com/digital-inspection-camera-67979.html

 

[NHGuy]

"The timing is set through the cam shaft and timing chain. The two dots are lined up" I don't know the 8.1 but on earlier big blocks & small blocks this is just where you set the sprockets when installing the timing chain. It's not TDC @ #1. You still have to bring the rotating assembly around while watching the valves on #1. When the #1 valves are both closed as the mark on the balancer comes to TDC you are at #1. Your timing could be off by 180 degrees if that is the case.

 

[05 sangerdlx]

The serial number is OW390234
I'll see if he's tried separating the harness and seeing if that starts it. So if the reluctor wheel is not in the center of the hole from the crankshaft sensor, could that be the problem? I don't see what else would be. I had the crank done in June so I doubt they will remember what they did to it.
He said that the boat is backfiring when it try's to start. So were assuming it's it of time somehow. But if the timing chain marks weren't lined up correctly wouldn't some cylinders have low compression due to them being out of time with the valves? All the compression test good on all cylinders. Is there any way to time this engine, or see if it is out of time?

 

[alldodge]

IMO if the gears were off by one tooth you could still show good compression but the spark may happen to soon

 

[05 sangerdlx]

I know we lined the dots up perfectly. But we started second guessing if the #1 piston was at top dead center after we put it all back together.

 

[05 sangerdlx]

Bump. Still haven't figured this out

 

[alldodge]

Bump. Still haven't figured this out

What have you tried or has changed since last postings?

 

[05 sangerdlx]

What have you tried or has changed since last postings?

Is there supposed to be a sensor on the front of the crank? I pulled the cam sensor off and can see the crank spinning. Was told there was a sensor on the front but can't see one. I see a hole for a dowel pin, or a dot, but that's it.

 

[Fun Times]

Is there supposed to be a sensor on the front of the crank? I pulled the cam sensor off and can see the crank spinning. Was told there was a sensor on the front but can't see one. I see a hole for a dowel pin, or a dot, but that's it.

The crank sensor is at the rear of the block... it's item number 7, http://www.mercruiserparts.com/Show_...ft+and+Pistons

The cam sensor is item number 12 at the front of the engine,
http://www.mercruiserparts.com/Show_...nd+Front+Cover

 

[alldodge]

Is there supposed to be a sensor on the front of the crank? I pulled the cam sensor off and can see the crank spinning. Was told there was a sensor on the front but can't see one. I see a hole for a dowel pin, or a dot, but that's it.

There are two sensors, cam and crank. The cam sensor is on the timing chain cover on the front of the engine. The crank sensor is on the rear of the engine. If you removed the Cam sensor and looked in you would see the Cam gear moving not the Crankshaft

 

[05 sangerdlx]

Bumping this back up. Trying to remember everything that has been tried so I'll list what I can remember:

Disconnected tach and gauges

Disconnected the entire wiring harness from hull of boat, hooked up to shops wiring harness and used shop fuel tank when at one dealership. Basically alleviated the boat as much as possible.

New crank shaft sensor.

Last shop said they tried a new cam sensor.

Pulled the engine and had the machine shop inspect the crank and check timing on the engine.

Last we knew the computer, ECM, was good. Pulled it off and put it on another 8.1 mercruiser and it started that engine. This was a year ago and two different mechanics ago so who knows now.

Engine is now back in and still not running.

According to two shops, the engine is getting spark and injectors fire intermittently and randomly. Not enough to start the engine, or really even try to start.

 

[Fun Times]

Bumping this back up.

Just a few different out of the norm thoughts is all but have a read starting with the fourth paragraph down on post # 3 about pulling current through the ECM base, http://forums.iboats.com/forum/engin...cm#post9971079

You may want to invest in a EFI Quick Probe to test the injectors, here's some videos about the probe in use, http://www.bing.com/videos/search?q=...DC13B8D3A4AD22

There's also a coil tester that is similar to the EFI probe, http://www.bing.com/videos/search?q=...CA124DDA533DDA

Could you please tell me how you made out if you received the old cam PS (the one NLA). Did it fix your problem for your 2001 - 496 Mercruiser ? Also I have my cam sensors now for the older apps, I have 3 different ones that is suppose to take care of all the NLA ones.

My problem now is I have to match them up with what reluctor is on the cam gear. This thru Merc went by engine serial numbers. GM / Merc changed the cam gear reluctor 5 times thru out the years. 2001 is different than 2002 and another change in 2004 came about then 2 more down the road I believe. The cam sensor has to match or it will not work properly.

Above is a quote from post # 13 in the following link regarding crank and cam shaft sensors and reading wheels mounted to the cam/crank shafts, http://www.offshoreonly.com/forums/g...001-496-a.html

More good info about Cam sensors,
http://www.offshoreonly.com/forums/...-expert-i-have-known-issue-3.html#post4336460

http://www.raylarengineering.com/vortec8100-camshaft-position-sensor-camshaft-gears.html

^ I see they pulled the engine to check the timing marks, did they by chance remove or replace any of the cam or crank parts during the machining job such as the reluctor wheel? Think we talked about this earlier in the topic, just wanting to be sure all the bases are being covered.

What type testing have you done to the wiring harness? mainly between the ECM and injectors as a starting point then check all power and grounds on the EFI harness. Since you didn't remove much of the wiring while R&Ring the engine, I'm thinking possibly one of the wires is chafed to ground while using a tool.

 

[05 sangerdlx]

So everything seems to test fine. Computer, wiring harness, changed out crank and cam sensors. Can fire all the injectors and coils with the mechanics computer/scan tool. The engine is just not getting an rpm signal. We took the crank sensor out to look at it and noticed that the groove the reluctor wheel cuts in the sensor is about 3 or so millimeters forward of center. If I remember correctly the original one, before grinding the crank, had the mark dead center. Could this be causing it to not get the rpm signal?

 

[Fun Times]

So everything seems to test fine. Computer, wiring harness, changed out crank and cam sensors. Can fire all the injectors and coils with the mechanics computer/scan tool. The engine is just not getting an rpm signal. We took the crank sensor out to look at it and noticed that the groove the reluctor wheel cuts in the sensor is about 3 or so millimeters forward of center. If I remember correctly the original one, before grinding the crank, had the mark dead center. Could this be causing it to not get the rpm signal?

Did the scan tool fire up the Tachometer too? It should be able to set the RPM up to 3000 RPM for about 10 seconds.

With the question about the crankshaft sensor making contact, I'm am reminded about a few earlier posts that may be of some interest to you including this how to service guide to remove, inspect and install the GM 8.1L Crankshaft sensor with inspection images... https://www.2carpros.com/questions/chevrolet-silverado-2002-chevy-silverado-crank-postion-sensor

Try doing the test in post 9, http://forums.iboats.com/forum/engi...ranks-but-will-not-spark-and-no-rpm-on-diacom

http://forums.iboats.com/forum/engi...-8-1ltr-s-efi-need-help?p=6669372#post6669372

Make your way to solution post # 41, http://forums.iboats.com/forum/engi...ing-when-the-key-is-turned-on-no-audible-beep

 

[05 sangerdlx]

Bumping up my old thread with some new questions.

Found the culprit. The reluctor wheel had a couple teeth that had a piece broken off. Guess I didn’t inspect it well enough on the install, and the machine shop I brought it too after it wouldn’t run didn’t catch it either. That was right after all the work was done and pulled the engine again to verify the crank was good.
Anyways, the engine runs now but when we plug the cam sensor in it runs like it’s out of time. The timing is on according to the shop. They say it runs ok until they plug the cam sensor in. What could cause this?

 

[alldodge]

I'm not sure how they set base timing without the cam sensor in, but that could be another thought for later.

In FT's post it list that the cam sensor mast match the reluctor, so do they match?

 

[Fun Times]

^ Also if the crank reluctor wheel was damaged/compromised then maybe the CAM reluctor wheel was too?
May want to find/rent a digital inspection camera to have a look inside.
https://www.harborfreight.com/catal...ured+Weight,f,Sale+Rank,f&q=inspection+camera

Though the following link is talking about a truck engine 'it's the same 8.1 concept...They talk about the sensor hitting the wheel and need to shim the sensor. Try taking the CAM sensor bolt out and see if the sensor moves while cranking the engine over ?.
https://www.justanswer.com/gm/6bz5p-gm-c7500-2002-chevy-c7500-8-1-gasoline-engine.html

The matching correct years is very important too for the sensors as mentioned in the previous posts above.

 

[muc]

Did I see somewhere that the cam sensor is from a truck?

Don't have firsthand experience with this particular part. But we have been seeing issues with non OEM parts. The 496 had some of the cam sensors go NLA but I don't think yours is one of them. Maybe try a MerCruiser cam sensor?