View Full Version : 454 Mag MPI backfiring

03-27-2016, 09:45 AM
Iím hoping to get some information. I have a problem I have not run into before.

I have a 1999 454 Mag MPI with 50 hours on it. It is stock with the exception of recently added Stainless Marine Exhaust. The last time I ran the boat prior to adding the SM headers I had white smoke or steam coming out of the starboard bank exhaust. I idled back to the ramp and put it up. I made some calls and consensus was a bad exhaust gasket. Or hopefully so. I had new headers coming in so I waited. I pulled and replaced the plugs. The plugs were crunchy plugs with deposits on them. OK, maybe I got some bad gas and it was from sitting up over the winter. I topped off with premium and added dry gas.

After installing the new headers (a two hour job), replacing the gimbal bearing and aligning the motor I went to run it. Now I have what seems to be a backfire in the starboard side. It runs smooth for a few seconds and then pop, smooth then pop and repeat. I ran it for about two minutes to see if it would clear but no.

If anyone has any ideas, I would greatly appreciate the help.

Pat McPherson
03-28-2016, 12:50 PM
Check or replace your cap and rotor then set the timing. You may also want to double check the firing order is correct; there are a couple of wires that are easily crossed.
You'll need a MEFI devise to lock the ECU and initial timing then use a timing light to check. I set both my 1996 and 1999 MPI/EFI engines to about 10deg advance of TDC at 1000rpm.

03-28-2016, 02:08 PM
Thanks Pat.
I hope it is something this easy. I took it to a boat shop since I don't have any MEFI tools. I can do anything with a car, but have yet to get something for the boat.

03-28-2016, 02:21 PM
You don't really need a tool to lock out the ECU. Just a jumper wire or paper clip to connect pins "A" and "B" on the data link plug.

For future reference, the attached DOC shows how you can easily make a code reader for your MEFI. I've used this to check fault codes on mine and it works great. It's also how I discovered you had to lock out the ECU to set the timing, like Pat said.

03-29-2016, 06:32 AM
Thanks Glen.
I built one of those for my 18 and forgot I had it.
Thanks for the doc. The trouble code chart will come in handy. I was surprised at how little is monitored then I remember this is an MPI. EFI motors have so much more which would be nice at the moment for me.
The more I read, I think I have a head gasket issue at best, cracked heads at worst. And some valve stems/seats issues.
The motor ran up to 2k fine while warming up without any backfires or roughness. So firing order is good.
Cylinder #6 had a wet plug and #5 has a white dry plug and the exhaust valve stem was white as in steamed. Funny though, white smoke came out the starboard side.
I think I'll take a look at what it takes to bullet proof this motor. The boat is fast enough and I prefer reliability.

03-29-2016, 08:08 PM
plugs don't look good one is way off from others
try to isolate what cly in missing pulling plugs wires then check that cly

04-03-2016, 07:34 AM
Thanks for all the replies.
I got the boat back from the shop and the backfire/popping was caused by swapped plugs wires. That will teach me to be more careful. But I did get a leak down test done to see where I was at on the heads.

Compression on all cylinders is 140.
Leak down is:
Port- 15/15/20/40
Stbd- 40/40/30/15

The tech thinks I have carbon buildup on the valves and from the looks of the plugs, I agree. They suggested seafoam in the tank and running it hard.
I ordered an OTC leak down tester so I can check it in the future. From what I've read the OTC is identical to the Matco tester for half the price on Amazon.

04-03-2016, 01:26 PM
Thanks for all the replies.
I got the boat back from the shop and the backfire/popping was caused by swapped plugs wires. That will teach me to be more careful. When I fired my 502 after swapping the cooling system, I put my hand on the exhaust and it was cold to the touch. Awesome. Thought I'd licked the overheating for sure. Put my hand on the other exhaust and it was hot. Damn! Water flow seemed to be equal, so that much of difference must mean I have cracked head or blown gasket (into the water jacket). I was just about to start pulling the heads and I noticed I had left all the plug wires for the one side off the spark plugs. Doh!!

They suggested seafoam in the tank and running it hard.That's going to be a fun repair! :yes: