Backfire under heavy rev

[justinf89]

Hey guys, I have a quick question. I've got a little problem on my mpi 502 volvo penta in my boat. The last few times I've taken it out, it doesn't seem to want to get up to speed, and backfires under heavy acceleration. I've replaced the fuel filters, cleaned out the 2 pumps(low/high pressure), lines, fuel cooler cell/float. Everything but injectors but I really think its a fuel pressure issue. Oh and I've changed all ignition components also over the past few months as maintenance. I was planning to take it out Wednesday but not if the problem persists. I started it up in the driveway today and it starts and idles fine, revs up fine, but not under very heavy acceleration(not under any load either). When I rev it hard, it hesitates and backfires through the intake once, then revs up fine. I just wanted to get an opinion before I go out and get a nice set of fuel pressure gauges, which I kind of always wanted anyway. Thanks guys.

 

[Chris142]

Originally Posted By: justinf89

I really think its a fuel pressure issue.

x2

 

[mechtech2]

Also, besides the obvious fuel starving possibility are: Valves could be sticky or not sealing right after sitting.. Ignition timing may be retarded.

 

[Brons2]

Originally Posted By: mechtech2

Ignition timing may be retarded.

Someone once told me something regarding ignition timing, If it's advanced it backfires through the mouth If it's retarded it backfires through the butt.

 

[Ken42]

Originally Posted By: Brons2

Originally Posted By: mechtech2

Ignition timing may be retarded.

Someone once told me something regarding ignition timing, If it's advanced it backfires through the mouth If it's retarded it backfires through the butt.

Thats one way of putting it.

 

[justinf89]

Ok so if it is caused by timing, what could be causing the timing to be off? This is a '98 model with electronic sensors and everything.

 

[mechtech2]

Originally Posted By: Brons2

Originally Posted By: mechtech2

Ignition timing may be retarded.

Someone once told me something regarding ignition timing, If it's advanced it backfires through the mouth If it's retarded it backfires through the butt.

Great general advice. Usually true. But ignition timing is only one possibility. A sensor, connection, or pickup ring may be faulty, and that would mess up the ignition timing. So could a TPS or out of whack MAF sensor.

 

[justinf89]

Ok guys, quick update. I got the el chepo fuel pressure gauge kit from harbor freight, which actually works perfect. I checked the pressure with the key on which was 42 psi(engine off). I then checked it with it running and it was still 42, I then revved it up to 2k rpm(warmed up a little first of course), and it dropped to 38 and held steady. I then revved it quickly to try and recreate the hesitation I was seeing yesterday, and nothing. I mean this thing ran so good at all speeds, so I think I'm going to take it out to load test it tomorrow. So is a pressure of 42 at idle good? Also, the lp pump was around 4-5 all the time when I did a seperate test.