1991 PCM 5.8 backfiring problem
Printed From: CorrectCraftFan.com
Category: Repairs and Maintenance
Forum Name: Engine Repair
Forum Discription: Engine problems and solutions
URL: http://www.CorrectCraftFan.com/forum/forum_posts.asp?TID=1872
Printed Date: May-15-2024 at 8:06am
Topic: 1991 PCM 5.8 backfiring problem
Posted By: Guests
Subject: 1991 PCM 5.8 backfiring problem
Date Posted: June-14-2005 at 1:56am
1991 Nautique Excel 351 Ford PCM engine; Motor will idle and run below 2000 rpm most all the day, but when run harder will intermittently start backfiring in the carb repeatedly and loosing power. Has been in several shops over three years' time without being fixed.
Any suggestions? One dealer suggested a complete PCM "Pro Tec" engine management system upgrade, blaming the 1991 version as beeing prone to similar problems all along.
Could a weak fuel pump cause carb backfiring?
Any other suggestions? I can manage nearly any diagnostic suggestions that can be offered.
Tom
Lake Lanier, Georgia
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Replies:
Posted By: Guests
Date Posted: June-14-2005 at 2:26am
Tom,
See this the topic: Holley carb "loading"
The board members here gave me a ton of good ideas and it sounds simillar to the problem I have stated to have with my 351.
good luck!
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Posted By: 64 Skier
Date Posted: June-14-2005 at 4:43am
I'd check the timing first on any engine that's backfiring. If the engine idles OK and then backfires when you hard throttle the timing advance is not working. Either that or the timing chain is getting sloppy.
The most important post on the other "thread" was SS201...IMO if you have an engine that's 10 plus years old...make damn sure the timing chain set is working or your gonna throw thousands at new ignitions etc. A new chain and gears are about $50.
After a few backfires, the Holley is known to rupture the Power valve and both the power valve and timing can be replaced and checked in short order by yourself or a mechanic.
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64 Skier
66" HO VTX and 67" HO Triumph
http://www.correctcraftfan.com/diaries/details.asp?ID=1071&sort=&pagenum=3&yrstart=1971&yrend=1975 - 71CC
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Posted By: Guests
Date Posted: June-14-2005 at 10:21am
This problem is intermittent... I've been told my PCM 351 has a double roller timing chain and it's very unlikely to be causing this problem.
Can anyone tell me the timing specs on this motor, also is there a jumper wire or plug that needs to be enabled to set the timing? (I have a Pro Tec electronic engine control, which means no distributor cap & rotor, just a flat plastic cover over the distributor shaft for the trigger module)
Next, do you think it could be the Holley carb if the problem is intermittent? Only one of four high speed runs will result in this backfiring problem.
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Posted By: David F
Date Posted: June-14-2005 at 12:29pm
The base timing for the 351 Pro-tec engine is as follows:
Std: 18 deg. BTDC @ 700 rpm, 30 deg @ 3600 rpm
HO: 18 deg BTDC @700, 26 deg @3600 rpm
You MUST remove the knock sensor wire to accurately set the timing. Removing the wire disables the cruise mode which is present at idle. Cruise mode advances timing when no throttle movement is detected after 30 secs.
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Posted By: nms1991
Date Posted: June-15-2005 at 1:29am
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Tom just check your timing at 3600 rpm, It should be at 30 degrees. I would suggest that you do check your carb to make sure it did not rupture the power valve. Teh problem does sould like it is more in the ignition than in the carb and the conversion kit for the Protec to distributor system is realitvly cheap like $600 which is less than the old Protec ignition modules went for.
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Posted By: nms1991
Date Posted: June-15-2005 at 1:30am
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I forgot to tell you make sure the knock sensor wire is connected before you check it at 3600 rpm.
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Posted By: ccwalker
Date Posted: July-04-2005 at 10:00pm
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Some 91 and 92's had computer modules that were poorly shielded. After much troubleshooting PCM finally admitted the problem. When I replaced this expensive componet it solved all the problems.
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