502 MPi - Still popping thru the intake. Need some ideas for troubleshooting.
09-04-2018, 10:55 AM
#15
Registered
What is your base timing? When I had my base timing set to zero, not knowing that it should have been 8 degrees, I would get a random pop through the TBI.
The timing was retarded too much causing the pop through the intake.
The timing was retarded too much causing the pop through the intake.
09-04-2018, 12:20 PM
#16
Platinum Member
Platinum MemberThread Starter
iTrader: (2)
Mike, I’ve thought that maybe the coil is an issue, but thought of them as either they work or they don’t. But I’ve seen some issues where the coil indeed has been the culprit.
I’m going to check over the wires to make sure nothing’s causing this on the secondary side (wires grounding etc). Perhaps it’s something with the MSD cap/rotor too (similar issue with an MSD cap but it was a Pro Billet on my S10).
Timing... she’s timed at 8 BTDC in service mode. I’ll check again this evening.
What leads me to think think that this is something ignition related is the way the engine runs when it starts messing up. The 454 in my 242 has an unknown to me collapsed roller lifter. The intake valve would heat up just enough to lose seal, and would get a very rapid fire pop. When it would happen, the engine wouldn’t really get rough though. Pop pop pop pop all in a very rapid fire, machine gun like fashion. Naturally, I’d shut it off, but it didn’t have a rough feel to it, and the engine would still accelerate. With the 502, you can feel it that it’s missing badly. I wish I could describe the sound. A coupe of times It’s done it, I’ve come off plane, rev’d it, and no more issue.
I’m going to check over the wires to make sure nothing’s causing this on the secondary side (wires grounding etc). Perhaps it’s something with the MSD cap/rotor too (similar issue with an MSD cap but it was a Pro Billet on my S10).
Timing... she’s timed at 8 BTDC in service mode. I’ll check again this evening.
What leads me to think think that this is something ignition related is the way the engine runs when it starts messing up. The 454 in my 242 has an unknown to me collapsed roller lifter. The intake valve would heat up just enough to lose seal, and would get a very rapid fire pop. When it would happen, the engine wouldn’t really get rough though. Pop pop pop pop all in a very rapid fire, machine gun like fashion. Naturally, I’d shut it off, but it didn’t have a rough feel to it, and the engine would still accelerate. With the 502, you can feel it that it’s missing badly. I wish I could describe the sound. A coupe of times It’s done it, I’ve come off plane, rev’d it, and no more issue.
Last edited by TomZ; 09-04-2018 at 01:38 PM.
09-04-2018, 12:59 PM
#17
Registered
Mike, I’ve thought that maybe the coil is an issue, but thought of them as either they work or they don’t. But I’ve seen some issues where the coil indeed has been the culprit.
I’m going to check over the wires to make sure nothing’s causing this on the secondary side (wires grounding etc). Perhaps it’s something with the MSD cap/rotor too (similar issue with an MSD cap but it was a Pro Billet on my S10).
Timing... she’s timed at 8 BTDC in service mode. I’ll check again this evening.
What leads me to think think that this is something ignition related is the way the engine runs when it starts messing up. The 454 in my 242 has an unknown to me collapsed roller lifter. The intake valve would heat up just enough to lose seal, and would get a very rapid fire pop. When it would happen, the engine wouldn’t really get rough though. Pop pop pop pop all in very rapid fire, machine gun like fashion. Naturally, I’d shut it off, but it didn’t have a rough feel to it, and the engine would still accelerate. With the 502, you can feel it that it’s missing badly. I wish I could describe the sound. A coupe of times I’ve done it, come off plane, rev’d it, and no more issue.
I’m going to check over the wires to make sure nothing’s causing this on the secondary side (wires grounding etc). Perhaps it’s something with the MSD cap/rotor too (similar issue with an MSD cap but it was a Pro Billet on my S10).
Timing... she’s timed at 8 BTDC in service mode. I’ll check again this evening.
What leads me to think think that this is something ignition related is the way the engine runs when it starts messing up. The 454 in my 242 has an unknown to me collapsed roller lifter. The intake valve would heat up just enough to lose seal, and would get a very rapid fire pop. When it would happen, the engine wouldn’t really get rough though. Pop pop pop pop all in very rapid fire, machine gun like fashion. Naturally, I’d shut it off, but it didn’t have a rough feel to it, and the engine would still accelerate. With the 502, you can feel it that it’s missing badly. I wish I could describe the sound. A coupe of times I’ve done it, come off plane, rev’d it, and no more issue.
A quick test would be to just go buy a replacement distributor from a local auto parts store and stick in it.
This is the one I have. It has a Marine SAE standard stamped on it. So it is a certified marine distributor. I did put a GM module in it, kept the original for a spare.
https://www.summitracing.com/parts/sum-850200/overview/
09-04-2018, 01:48 PM
#18
Platinum Member
Platinum MemberThread Starter
iTrader: (2)
Both have the standard GM Delco EST distributor that’s used with the Merc EFI/MPI. ive thought about rebuilding the distributor (module and pickup with a new cap and coil) to see if it would make a difference. I didn’t think the module did much once the engine was started and the MEFI ECM took over, but the pickup could be a potential culprit.
There is still the issue of a bad alternator here too. I’m running both engines off of this engine. I’ve noticed too that when this starts happening, the tachs seem to jump around a bit (especially the port engine). At higher speeds (above 4 k) the tachs seem to shake sometimes with upwards of a 2-300 rpm swing. I started noticing this at the end of lost season on the starboard side, now both do it. Defnintley something funky going on.
There is still the issue of a bad alternator here too. I’m running both engines off of this engine. I’ve noticed too that when this starts happening, the tachs seem to jump around a bit (especially the port engine). At higher speeds (above 4 k) the tachs seem to shake sometimes with upwards of a 2-300 rpm swing. I started noticing this at the end of lost season on the starboard side, now both do it. Defnintley something funky going on.
09-04-2018, 02:11 PM
#19
Registered
iTrader: (1)
Get your electrical issues straightened out first.
Always,always repair known issues before you start throwing parts and energy(too much thinking) at possible non issues.
Always,always repair known issues before you start throwing parts and energy(too much thinking) at possible non issues.
09-04-2018, 02:52 PM
#20
Registered
Fuel pressure is easy to test on these motors and if it is low can cause all sorts of issues. I was chasing a surging idle/hard warm start issue on one of my 2000 502MPI. Motor ran fantastic and was always stronger of two. Threw TPS/IAC at it and no change. I was also having some backfiring occur from what I thought was other engine.
Fnally bought a $22 gauge for schrader valve and found regulator was bypassing at 30 psi static. Also found other motor was at 36 psi static. Instantly fixed my starting issues, idle issues, and no more backfiring.
It was impressive how well they "sounded" on top end so I discounted having fuel pressure issues and originally leaned to the electronic side of things. Wrong thinking there for sure. Fuel pressure is one of first things I check now if I or a friend have issues.
Fnally bought a $22 gauge for schrader valve and found regulator was bypassing at 30 psi static. Also found other motor was at 36 psi static. Instantly fixed my starting issues, idle issues, and no more backfiring.
It was impressive how well they "sounded" on top end so I discounted having fuel pressure issues and originally leaned to the electronic side of things. Wrong thinking there for sure. Fuel pressure is one of first things I check now if I or a friend have issues.
