Backfiring Engine under load

[davebuc]

Hi, Would love to go boating...but...backfiring problem

1999 Ski Nautique
Gt 40 engine, PCM 1.23 :1 transmission

This winter, I had the engine rebuilt. (3500 hrs on previous one)
Engine (machine) shop put it back together up to the intake. They put distributor in, I installed plugs, wires, fuel injection parts, Computer, electrical.

I bought my new cap, rotors & plug wires from nautiqueparts.com
For spark plugs I used Autolite Platinum 2.

I set the timing @ the lake per instructions: 5 degrees BTDC in neutral @ 2000 RPM with the SPOUT plug removed

At the lake, in neutral, the engine runs well and responds as expected 600 - 4000 Rpm.
I am not quite sure the bracket that holds the throttle cable and bolts to an intake bolt is in exactly the right position. By tweaking it slightly by hand, I can change the idle down to where the engine stalls out, or alternatively I can increase it over 1000

If I idle at around 700-750, When I shift into forward gear, it wants to stall. If I give it a little bit of gas, I can keep it running.

If I slowly increase the throttle, things seem normal until about 1100-1200 RPM. At this point, I get backfiring.

I do not have the black rubber boot (distributor cover) installed yet. I was waiting until after I set timing. A local mechanic suggested this may be my problem, as arching was "a known problem with that distributor, and the rubber boot is to reduce / eliminate that"

Transmission (in case this is relevant?) The transmission is basically new. Got a new one that had been sitting around for a few years at a price that worked for me. Seems to work fine. The painting plastic plugs were still in it. Sadly I broke the input shaft on my old one which worked great by pulling the engine up too soon when removing the engine.

While driving the boat, I tried manually adjusting the distributor timing by hand with the SPOUT connected. There was no position for the distributor within the reasonable running range that improved the 1100-1200 RPM backfire.

I also tried giving it more throttle when it hit 1100 and it seemed like it really did not want to open up and go.

Last winter, (while disassembling the engine prior to rebuild) I accidentally grounded the main battery connection on the bottom of the computer to the engine block for less than 1 second. Did not expect it to be live with ignition button not on. My bad. Would this have damaged my computer causing my current backfire problem?

Tomorrow, I will drain the fuel out of the FCC to see if it happens to look watery and investigate the "small clear hose" inside the FCC that is reported to crack occasionally? I read a post where someone was getting backfires for about anything above idle when this hose was cracked.

Any suggestions as to what I should investigate? It seems odd that the backfiring only happens under load.

Many Thanks for reading.

Dave
Springdale, UT

[Mikeski]

Vacuum leak?

[LakeRats]

Buy a compression tester. Check compression to verify valves set correctly. Re verify timing.

[Mamigacz]

The first thing I would check is your firing order (1 - 3 - 7 - 2 - 6 - 5 - 4 - 8). If two wires are on the wrong distributor posts, you could experience missfires. Some people might argue that it would backfire at idle also if this was the problem, but it depends on which wires are crossed. I already had two wires on the wrong distributor posts, and the engine ran pretty decent at idle (better than I expected after finding the problem). However, it had a notiable lack of power under load.

The other thing to check is to make sure that all of the injector pigtail connectors are going to the correct injectors. I'm not sure if its possible to mess this up, but it wouldn't hurt to double check.

[nms1991]

You probably have a bad pickup sensor in the distributor, but first remove the distributor cap and use compressed air and blow out any particles near the sensor out and retry the engine at the rpm where you are having the missing problem.

[davebuc]

A bit more info:
-forgot to mention yesterday that in there are 2 square corners on my distributor (see picture below). One of them was running into my intake and preventing me from rotating it far enough to get it to 5 deg btdc in neutral @ 2000 rpm (w/ spout out). The only way I could get 5 deg was to rotate all the plug wires around 1 spot on the dist counterclockwise. I don't know if it is a good or bad idea to round off a corner so that I can turn the dizzy farther.

I also found the distributor cap was a bit wiggly. I could have screwed the cap on tighter when I installed. Perhaps this is part of the problem.

Today in driveway:
-I replaced the short fuel hose inside the FCC because it was linked to similar problems in forum posts. Didn't seem in super bad shape

-Fuel inside the FCC looked good. No evidence of water / milkyness.

-Checked all the Fuel injector pig tails, they all seem to be going to the correct places. I also snugged all electrical connections I could find.

-I checked the fuel pressure. It is 34 psi @ both the top of the FCC and the Starboard Fuel Rail. Info plate on the engine says it should be 39psi +/- 3psi, so it is a bit low.

-I removed all the plug wires, put number stickers on both ends and reinstalled them with the firing order Mamigacz gave me above.

[davebuc]

In case this is also worth mentioning, when the machine shop did the engine rebuild, they encouraged me to switch from a flat tappet cam to a roller cam so I did, and they also switched all the other necessary hardare.

[davebuc]

It has been revealed that I do not have the correct spark plugs. I need Autolite 764. The Double Platinum APP 104 I have crosses to Autolite 763.

[TRBenj]

It has been revealed that I do not have the correct spark plugs. I need Autolite 764. The Double Platinum APP 104 I have crosses to Autolite 763.

Like I explained on CCF, the 763's were the correct plug... until they discontinued them. The 764's are now what everyone is running instead. The APP104 crosses to the 764 per Autolite's site, so its not a compatibility issue... though I would still stick with the 764's next time. Not only do the low end copper plugs perform better, but theyre cheaper. The platinums will last longer, but arent as good a conductor as plain old copper, and will probably need to be replaced due to external corrosion concerns long before they would otherwise need it (so theyre a waste of money).

I would put more stock in the loose distributor cap causing at least part of this issue.

No need to cross post both sites in the future at the same time, perhaps.

[lucky7t]

From the looks of it your engine is a ford.

Two things I would do ASAP.

1. New ignition module ( these are very tricky as some will shield emi and some won't) if you can afford it then buy two from napa. Get the gold series. We have this issue constantly. Emi is electrical field from your alternator , wires etc...

2. Distributor pick up ( these two have issues)


If I were a betting man if you replace these you will be running great.

[OcoeeSki]

Sounds to me like your distributor is a tooth off, it's not hard to do when reinstalling a distributor if you don't double check yourself. You should have plenty of room one way or the other to adjust the distributor to 5 deg, and for sure you should not need to rotate the plug wires. If you manually turn the motor to tdc of #1 and make sure the rotor points to the correct plug on the rotor cap you should be close then put the timing light to it. The backfire under load makes sense to me because the computer advances the timing due to loads determined by the various sensors, at idle those loads do not exist. The computer is advancing electronically past what is mechanically possible.

[Mamigacz]

I agree with OcoeeSki with the distributor being a tooth off. Its definitely not ideal to have it tooth off because the plug wires need to be indexed, sometimes the plug wire lengths don't work out and the electrical harness is rotated to a position that is not ideal.

However, as long as your initial timing is set a 5 degrees (per spec), there should be no initial timing issues (even if its off a tooth). The pick up coil does care what cylinder is getting spark (if its working properly). The distributor cap orientation is what tells which cylinder is going to get spark. After the inital timing is set the ECM will advance the timing based on engine speed, knock sensor data and engine temperature.

Are there any engine codes?

I know this does not sound like the safest thing to try, but it would be interesting to put a timing light on the harmonic balancer while you accelerate in gear. To see how much it is advancing.

[davebuc]

Thanks Guys-

Where is my ignition module?

You were right that my distributor was off by a tooth.
I replaced the distributor yesterday

Here is the update about today's failed lake test (which I cross posted to CCF).

Update:
-checked condition of fuel. It is not watery, not milky. It looks like gasoline
-Installed Autolite 764 sparkplugs
-verified firing order 1 - 3 - 7 - 2 - 6 - 5 - 4 - 8 CCW on dist cap
-installed remanufactured distributor from Rock Auto (part # 302891) by A1-Cardone (cap, rotor & wires are also new). I did this b/c dist pick-up was a possible cause of the problem

-distributor cap is now snug on distributor (was a bit loose in previous lake test)
-bolts on upper plenum of intake were a bit loose, so I tightened them

Went to lake for a test.
-re-verified engine timing w/ SPOUT removed, set to 5 deg BTDC @ 2000rpm
-reinstalled spout

Went for a test drive. When I put it into gear, it stalled again. Restarted fine. On the second try going into forward, I gave it more gas, and it didn't stall.

Idled fine to edge of no wake zone. Slowly added throttle leaving the no wake zone. Got 1 backfire around 1200rpm (previously multiple backfires happened at this point).

At backfire, I left the throttle @ 1200 for a few seconds. No additional backfires. After 15 seconds at 1200, I added more throttle. At after about 10 seconds at 2000 rpm, another backfire occured, then a squeal (lasted 3 sec total). It sounded mostly, but not completely like a belt squeal. I backed off the the throttle, and the engine completely cut out while I was backing down the throttle. I check belt tightness w/ engine stopped. They seemed to be the same tighness that they ran well at all season.

When I looked at it, there was some exhaust smoke coming out of the flame arrestor.

Check engine light was on.

I let it sit for a few minutes.

I tried to restart it. I would crank, but not ignite. It sounded like no spark was igniting.

It doesn't make any strange noises while cranking.

At home, I tested spark with a gap tester that allows 4 plug wires to be hooked up at a time. No spark port, No spark Starboard. I then pulled the coil wire off the top of the dist. cap and hooked it up to the spark gap tester. It did not spark. I verified my ground that I used for all the spark tests before I did them by touching a multimeter to it and the positive post on my battery. The ground was good.

So, no spark at coil now. This is new as of today.

There are 3 red circuit buttons next to the ECM. The top one sticks out farther than the other 2 (the 60 amp sticks out farther). I don't know if this was always this way. I have noticed it since the engine rebuild when I reinstalled the ECM. If I push on it (60 button), it does not push in, nor can you pull it out. It seems to have less spring than the other two below it.

I have verified compression in all 8 cylinders with all 8 spark plugs out at the same time. Here were the readings:
1. 150 5. 145
2. 155 6. 145
3. 145 7. 145
4. 150 8. 145

I looked for in line fuses, wondering if one had blown. I found several in the dash, which were ok, but none really in the engine compartment.

Any ideas what this is pointing to?

Thanks for the help so far,
Dave

[davebuc]

I found the Ignition sensor. It is mounted up top next to the coil.

[Paxdad]

It sounds like the timing gear set was not indexed correctly causing the valve timing to be way off. I would go back to the engine builder.