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Old 11-30-2013, 12:28 AM
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Mark rsa2au Mark rsa2au is offline
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Misfire through throttle body at 4500rpm??

Hi Guys

I have been chasing a few issues with my boat since I got it and slowly I am getting on top of it. Boat is 2001 X5 with a 5.7 TBI indmar.

The current issue is a severe misfire back through the throttle boddy at 4500rpm. The boat pulls like a freight train untill then and is still accelerating hard untill the misfire. (it is not the rev limiter which is at 5200rpm).

Running the boat on the lake with the cover off and flame arrester off, the noise sounds like the fuel in the intake is igniting. It fires off very loudly 2 or 3 times and you can see the vapour being pushed out of the throttle body even at 4500rpm. The ECM then pulls revs out.

There are no codes. I have pulled the rocker covers off to check for issues with the valves and I can see none. I have not done a compression test yet as there is plenty power and uses no oil.

So far I have replaced the Fuel pump, lines, filter, Injectors, and regulator. PSI is stable at 30 across the rev range untill at WOT it increases slightly as expected. fresh fuel - non ethanol.

I have also replaced the plugs, plug leads, distributer cap, and rotor.

The temp sensor tests perfect, as does MAP sensor, and faulty TPS has been replaced. Faulty Knock sensor has also been replaced.

Any help would be appreciated as my next option is a 8 hour return drive to a Malibu dealer on the water with an Indmar diognostic computer. They have agreed for me to hire their tech team for the day on the water to try to find the issue as it occurs under load on the water. 2 techs x 8 hours, my travel, and accomodation = BIG $'s.

Thank you!!
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Old 11-30-2013, 12:37 AM
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How old are the spark plug wires? Possible cross-fire - run engine in the dark you may see sparks on spark plug wires. The wires are the only parts I don't see on your list.
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Old 11-30-2013, 01:06 AM
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Mark rsa2au Mark rsa2au is offline
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Sorry Mate, Plug leads = Plug wires. less than 20 hours old. Did it with old leads/wires as well.
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Old 11-30-2013, 01:21 AM
T-Rager T-Rager is offline
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I would perform a compression test. I suspect a valve-related problem. Perhaps a weak or broken valve spring.
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Old 11-30-2013, 02:49 AM
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Quote:
Originally Posted by T-Rager View Post
I would perform a compression test. I suspect a valve-related problem. Perhaps a weak or broken valve spring.
When I took the rocker covers off, I noted one valve spring had some surface rust. Did not think this would affect it though?

It seems weired that it is at only 4500 rpm and up, so I thought electronics not hardware, but then if I knew what it was I would have fixed it already....
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Old 11-30-2013, 06:50 AM
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Quote:
Originally Posted by Mark rsa2au View Post
When I took the rocker covers off, I noted one valve spring had some surface rust. Did not think this would affect it though?
It wont. It's just from sitting up and moisture in the valve cover. Pretty normal for any engine that sites up for a few months.
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Old 11-30-2013, 06:54 AM
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My old chevy truck does this and it's a distributor problem. Mine does it under load at about 1100 rpm. If I replace the cap and rotor it's fixed for about a month then back again. Pull your cap back off and check for play in the distributor, it's possible the drive gear is worn and at that rpm it's moving just enough to jump across to a different lug on the cap. I want to say this is a fairly common problem on a 4.3 vortec motor in chevy blazers. But I can't remember for sure.
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Old 11-30-2013, 09:16 AM
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If it's possible to run it at this speed at night, leave the engine cover at the dock and have someone watch for arcing between plug wires and at the coil. This could indicate bad grounding for the distributor- the current will take the easiest path to ground and if the plugs are gapped too wide, it could cause this.

Loosen the distributor hold-down bolt, slide the small plate away and clean the surface where it digs into the intake manifold, clean the groove where the base fits into the manifold and make sure there's no rust. If it has rust, scrape it clean and vacuum anything that's loose before measuring the resistance from the distributor to the manifold- you should see zero Ohms. If you see more than this (it's important with low current), it might be a good idea to scratch a mark for the placement of the distributor and rotate it a few times, to scrape it clean before re-aligning the marks and tightening the hold-down.

Make sure the screws that hold the IC module in place are clean, inspect the inside of the distributor cap for carbon traces (they'll look a bit like someone drew on the under-side with a pencil) and do a compression/cylinder/vacuum leak down test. Compression test is good for making sure it's even across the cylinders, but the leak down test tells more- if it leaks down quickly, it could be worn vales/seats. If an intake valve or seat is worn or cracked, it could cause hot combustion gases to ignite the fuel in the intake, above the valves.
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Old 12-05-2013, 07:34 AM
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Mark rsa2au Mark rsa2au is offline
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Hi Guys, thanks for all the advice so far!!

Been so busy I did not have time to check out the boat untill tonight . I have removed, wire brushed, and tested every earth connection I can see. All tested perfect before and after. also tested from battery - engine block - earth connector on front of motor and the rear connector, all 100%

I then tested the earth on the distributer and it was perfect, but there was some corrosion building up so I removed the bolts nuts etc, wire brushed to bare metal and replaced, and it all tests fine. There were no carbon traces inside the distributer and it is about 30 hours old so still clean.

The only area I suspect could be an issue is the Ignition controll module (see below - pre clean). there was a lot of corrosion on the mounting screws, but they still tested fine to earth. Anyway I removed, wire brushed the screws, and sanded the holes on the ICM. Before replacing I smeared Dielectric Grease on the underside of the ICM and screwed tight. (Chevy forum recomendation)

I do not have access to compression or leakdown testers so I will have to take it to my local garage next week and they should be able do it for me.

Looking at the picture below is this a standard GM ignition module? Can I go to an American car parts store and just buy one off the shelf? I do not know of any parts store that will test these in Australia so I can only replace it.
Name:  ignition module.JPG
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Old 12-05-2013, 08:09 AM
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Yes, a regular automotive part should work.
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