1991 PCM 5.8 backfiring problem

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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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ccwalker Members Profile Send Private Message Find Members Posts Add to Buddy List Newbie Joined: December-18-2003 Status: Offline Points: 11	Post Options Post Reply Quote ccwalker Report Post Thanks(0) Quote Reply Topic: 1991 PCM 5.8 backfiring problem Posted: July-04-2005 at 10:00pm
	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.

nms1991 Members Profile Send Private Message Find Members Posts Add to Buddy List Groupie Joined: May-31-2004 Location: United States Status: Offline Points: 46	Post Options Post Reply Quote nms1991 Report Post Thanks(0) Quote Reply Posted: June-15-2005 at 1:30am
	I forgot to tell you make sure the knock sensor wire is connected before you check it at 3600 rpm.

nms1991 Members Profile Send Private Message Find Members Posts Add to Buddy List Groupie Joined: May-31-2004 Location: United States Status: Offline Points: 46	Post Options Post Reply Quote nms1991 Report Post Thanks(0) Quote Reply Posted: June-15-2005 at 1:29am
	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.

David F Members Profile Send Private Message Find Members Posts Add to Buddy List Platinum Member Joined: June-11-2004 Location: United States Status: Offline Points: 1770	Post Options Post Reply Quote David F Report Post Thanks(0) Quote Reply 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.

Tom Members Profile Send Private Message Find Members Posts Add to Buddy List Guest	Post Options Post Reply Quote Tom Thanks(0) Quote Reply 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.

64 Skier Members Profile Send Private Message Find Members Posts Add to Buddy List Senior Member Joined: February-08-2004 Location: United States Status: Offline Points: 415	Post Options Post Reply Quote 64 Skier Report Post Thanks(0) Quote Reply 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.
	64 Skier 66" HO VTX and 67" HO Triumph 71CC

keuka1 Members Profile Send Private Message Find Members Posts Add to Buddy List Guest	Post Options Post Reply Quote keuka1 Thanks(0) Quote Reply 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!

Tom Members Profile Send Private Message Find Members Posts Add to Buddy List Guest	Post Options Post Reply Quote Tom Thanks(0) Quote Reply 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

1991 PCM 5.8 backfiring problem

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