2.0 L D-Jet Missing

[DNHunt]

I have developed a miss at 2700-3200 rpms going downhill in 3rd, 4th and 5th when the car is fully warm. It is not consistant but happens at least half of the time. Also on compression I am noticing new sounds behind me, certainly a rumble possibly a some backfiring. The muffler is a relatively new Bursch.

FI is stock D-Jet, ignition is MSD 6A with Pertronics in a stock dizzy.

Timing is at 27 degrees at 3500 rpms. Plugs are Bosch W5CC gapped at .041. Idle is 900 rpms and the car is very responsive with no ignition problems except as explained above. Valves are at .006 for intake and .008 for exhaust.

It seems to me this would be the leanest part of the A/F curve so I am assuming it is getting to lean. As a first step I clamped off the decel valve. I saw no change. I would appreciate any ideas.

Thanks, Dave

[pbanders]

When you say "going downhill" are you talking about coasting with the throttle closed? And if so, is what you're hearing a light backfire?

[DNHunt]

Yes, coasting and yes a light backfire.

[pbanders]

OK, then one more question. Is your 2.0 L a 73-74, or is it a 75-76. Makes a difference in the answer.

[Bruce Allert]

Ya know... there's been quite a few of these symptons lately (mine included).
Me'thinks there's a gremlin about
I've been replacing parts (like they're going outta style) that are supposed to have taken care of my misses... the ones in my car
but the misses gremlin just got too gooda holt of me!
but I'll keep try'n

[DNHunt]

Brad; My car is a 74. I may have misled you. The miss happens when I coast but also when I give it a slight amount of fuel to maintain my speed.

Thanks,

Dave

[Paul_Heery]

I'm not Brad (nor do I play Brad on TV), but until he wakes up and chimes in I'll give you my opinion.

I have a '74 2.0 and was experiencing similar problems. It happened only after the car had warmed up and became very repeatable. With the car moving, with the car in gear, when I lifted I would get a light backfire.

In my case, it turned out to be a vacuum leak. The PO had installed a mechanical VDO vacuum gauge. I guess that he wanted to monitor what was going on in the FI system. Well, it turned out that by the way he spliced the T into the line he caused the leak. I disconnected the gauge, fixed the leak and the problem went away. Anybody have a block of wood I can knock on?

[hardflex]

It could be that your throttle position sensor is not adjusted correctly, so when you open the throttle slightly it's still on the idle circuit, which is a fixed amount of gas, then you allow more air in without coming free of the idle circuit. Thats what the adjustment does, when the throttle comes off completely closed (idle circuit) it transfers to the ecu to supply fuel based on the other sensors.

There is a procedure in the tech section

[3D914]

Dave,

Not that this has anything do with your specific problem, but I noticed your sparkplug gap appears larger than spec. Is this required for the MSD ignition, as my manual says something like .028?

Just curious.

[pbanders]

Like others said, could have been a vacuum leak. What I was suspecting was that it was due to a lean idle mixture, as set by the ECU knob. When the throttle is closed and you're coasting in gear, the mixture is set to the idle condition. If lean, you get some light backfiring. It's an easy thing to check - just try clicking the ECU knob 1 or 2 clicks clockwise. If it goes away, that's what was happening. No? Turn it back to where it was and start looking for vacuum leaks.

[DNHunt]

Messed with idle knob on ECU yesterday. It didn't help so I'll try to find a vacuum leak on Saturday if the weather is not too bad.

Thanks,

Dave