Missing at low idle on a couple of cylinders

Greetings all. I have a problem with my 308 QV (1983) that I have been working with a mechanic on and wanted to theorize a bit. I have owned the car for a couple of years, and have been driving it (when I can).

Symptoms:

Issue 1) Missing at low idle (which has been set up around 1500RPM, and this is too high) (once I'm above 3k during normal driving it seems to clean up), and during warm up cycle.

Issue 2) It stranded me a few months ago after a long run and would not start. The previous mechanic I took it to believed the cylinders had been flooded. They found low cylinder pressure but were able to get pressure back. They were unable to diagnose a cause, but I did need a new battery (replaced).

The fuel distribution system has been replaced and it is correct for a 1983 QV (the non-lambda one). It has been replaced twice in the last 4 years actually (once as documented by the prior owner, and once by me when I was trying to get it through emissions). This leads me to believe that is is less likely (though not impossible) for the distribution system to be at fault. This is an issue the car feels like it has had since I have owned it, so I believe the symptoms might be lending themselves to make it look like a fuel distribution issue.

Current state:
When I start the car up, it goes through the warm up cycle, which sits at about 2.5k-3kRPM (again, slightly high). At that RPM during warm up if I put my hand near the exhaust on the right side I feel an uneven pulse of exhaust, which my understanding is a likely indicator for missing. On the left side it feels more even, and I can tell on cold mornings by the amount of condensation burning off that the left side is pushing more air out. Additionally, judging by the sound there is an occasional but not quite regular miss happening. When I stop at a light or stop sign, and I go to idle, I can hear missing start up, though it tends not to be enough to stall me, and I feel some sputtering below 2kRPM as I begin to accelerate again.

This leads me to believe the missing is electrical/ignition. The plugs and cables were replaced less than 18 months ago, along with both distributors and caps. The coils were not replaced as far as I'm aware.

So, my question (thank you for reading), is: Is it possible for the coils to be the root cause for the missing? Any other suggestions for "easy" things to look at? I bought a new fuel and air filter, but because it's a specific bank of cylinders (the back ones of course) I would not think something that covers both would be the root cause.

Thanks!

 

I am running OEM caps/wires, and they were replaced about a year and a half ago. I too believe it is dropping a cylinder (at low idle at least) and then eventually flooding out. At higher rev's it seems better, or at least slightly less missing. I hadn't thought about checking it in the dark, that's a good idea and I will try that this weekend!

 

We checked one of the plugs in the easy to reach bank and it looked "normal". I feel like perhaps the idle was set higher because the missing tends to happen less at higher RPM. It is eventually flooding though even at higher idle. The real check will be when we can get to the ones in the back.

 

Is it possible for exhaust to be coming from both pipes if only one bank is running? If so that's a possibility.

 

I presume it's the same with 308 then? If that's the case is it possible to just swap the leads from one of the ignition coils to the other and see if the problem moves?

 

Update: We swapped the digiplexes to see if one was causing an issue, and the problem didn't seem to move. The spark plugs on the trunk side of the motor seemed fine, but those are not the ones that seem to be missing either.

Checked vacuum on a few common places to confirm no obvious holes/tears in tubes.

Validated that the fuel distributor was replaced in 2022 and then again in 2023 (after I bought it) because it was running really rich and I suppose in both cases the presumption was that the fuel distributor was causing that issue. However we noted that no one had replaced the fuel injectors themselves.

So now I have fuel injectors on order. The theory is the car is 42 years old and perhaps the springs on a couple of injectors have loosened enough that they are passing fuel at lighter pressure than they should be, and that seems logical to me considering all the fuel related issues I seem to be having. I also ordered injector carriers in case any of those are bad/split/torn/broken or otherwise not functioning.

If that solves it I will post again.

 

Yep! That fuel distributor. It was installed, tested, and adjusted. Twice! (So we don't think that's the problem). We will be checking (all) the plugs/cables when we are able to get to the cabin side of the motor (not this weekend but probably next weekend). For the springs leaking theory that's good to know. The cylinders that are missing are definitely not being starved, maybe they are dirty, though I'm not sure I understand if they are open all the time how that might contribute to too much fuel. It would be too little I would presume? Maybe that's not a correct assumption.

Regarding the WUR - wouldn't that affect all the cylinders? It is missing both during warm up and after warm up, but it's especially noticeable at low(er) idle.

One part that I found as part of the injection system that also hasn't been discussed is the injector carrier. That is supposed to help aerosolize the fuel. If one of those is broken or split maybe that's a problem? That's going to be more difficult to get to than the injectors themselves though.

We haven't pulled them, but they were replaced about a year and a half ago too. It's something we can check when we get to the cabin side of the motor.

We checked the distributor cap on the cabin side and it seemed pretty clean. The distributors were both replaced about a year and a half ago when it wouldn't pass emissions too, so in theory they should be good, but that's something to look at too.

Thank you all for the ideas! I'll keep you updated.

 

We can check continuity on those as well once we get to the cabin side of the motor. Thanks!

 

We swapped the digiplexes but the missing didn’t seem to change. The sensors are on the list too though.