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Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

Just looked back over the list of repairs done by the PO. Injectors were not replaced. Is it worth putting in a new set while I'm pulling them out anyways?
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1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6

924RACR · Posted: Wed Nov 12, 2025 9:44 pm Post subject:

Sure sounds like it.
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Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Riggard · Posted: Thu Nov 13, 2025 7:02 am Post subject:

The seals I would definitely replace. I replaced mine, and to get the injectors out, I typically use a wrench/pliers. If yours are hand tight, airleaks are probable.

Injectors is not a must-do, but something I would recommend. They get dirty internally and/or rust. Even getting them cleaned ultrasonically might not help.
You can check your current ones by having them spray in a jar and checking the spray pattern, it should be fine mist. Often it's more of jet washer.

I replaced mine a while ago because not all of them had a good spray pattern, it really worked wonders for how the engine ran. Much smoother and fewer vibration.

Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

Bosch still the go to brand for injectors?
_________________
1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6

924RACR · Posted: Thu Nov 13, 2025 10:25 pm Post subject:

Definitely Bosch - original manufacturer, of course.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

Riggard · Posted: Sat Nov 15, 2025 5:57 am Post subject:

didn't even know there were other manufacturers. Went with the Bosch's, think they were like 30 each from monointegrator.com

Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

Installed the new injectors and seals today. Of course there was drama. The injector on No. 4 was leaking a lot of fuel around the seal. I mopped up the gas, pulled the injector and re-installed it fully expecting that it would still be bubbling fuel. But no further problems... I must not have seated it properly first time around. Definitely the original injectors and seals. Seals were mushroomed and hard as a rock. I was actually able to pull the No. 2 injector with my fingers.

Idle has smoothed out and throttle response is improved. Prior to, when accelerating hard and the rpm would approach 6k, it would always miss somewhat and seem to be straining a little. Not any longer.

But damn if my idle still wants to hold around 1200-1300 rpm. If I'm at a stop I can hold the brake and let the clutch out just enough to pull the rpm down, the idle will hold at about 950. Occasionally it will decide to drop to 1100 and hold on it's own. I'm stumped. All the vac lines are good and properly connected. Injector seals are new. This high idle did drop a couple hundred rpm so I'm chipping away at it...

What should I check next? Distributor?
_________________
1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6

Raize · Joined: 18 Sep 2013 Posts: 426 Location: Scotland

You could lean it out a bit to make it idle lower using the mixture screw on FD if you are not super fussy about emissions.

Assume idle screw on throttle body is already fully in.

If your AAV isn't closing fully (or close as) when hot then that might be giving a high idle.

Finally, there is just a design flaw that gives high idle - the 3 way hose that goes between brake booster, intake manifold, and the K-Jet rubber boot it creates a pretty huge throttle bypass. You may find that if you squeeze the K-Jet rubber boot leg of the hose flat with some pliers the idle will drop and the car may even stall. I don't know if maybe there's meant to be a restrictor or something in there. Parts catalog doesn't show anything though.

Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

I actually pulled the AAV while I was doing the injectors. I had cleaned it some months ago but I confirmed that it is working with an oven and freezer test. When the valve is at approx 80*F, it's almost completely closed (only a very hairline sliver of light can be seen through it). After 15 min in the freezer the slit had opened to about 3mm.

My car doesn't have the 3 way hose. There's a thingamabob that is mounted to the intake that those hoses route through...

(not my pic or engine).

Is this a deceleration valve?

I also re-set the timing today, fiddled some with the FA mix, set the idle screw ... working back and forth between those until I had the idle at a solid 900. Took her out for a drive and when I got to the first stop sign, the idle was back at 1200. With my foot on the brake I can release the clutch, pulling the rpm down and it will stay a hair under 1000 rpm (this seems to work consistently now, prior to there was a half chance the idle would creep back up).

So I'm contemplating two possibilities: 1) weights in the distributor are a bit sticky and/or 2) I have a sticky deceleration valve (assuming my car even has one).

Thoughts?
_________________
1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6

fiskolja · Joined: 08 Aug 2024 Posts: 21 Location: Sweden

Yes, that's the decel valve.
Does the rpm stick when you let off the accelerator? If so, it could be at sticky decel valve.

>With my foot on the brake I can release the clutch, pulling the rpm down and it will stay a hair under 1000 rpm

I'd rather have a slightly high idle than cause unnecessary wear on the clutch. Does the RPM drop if you blip the throttle?
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924 NA 1977

Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

When I take my foot off the accelerator, the rpm slowly drops to 1200.

I've only done the "release clutch with brake on" maneuver to see if the idle would drop or if it would climb back up. I've only done it 6 or 7 times as a test.

Can that decel valve be cleaned with carb cleaner or some such?
_________________
1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6

Cedric · Joined: 27 Aug 2004 Posts: 2776 Location: Sweden

Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

Well it's not the deceleration valve.

I cleaned it and it seemed to work. The valve would open at 20hg of vacuum and close fairly quick. So I completely disabled the valve (putting a vacuum plug where the lines for the valve connect at each end and bypassing the vac line that actuates the valve). Still getting the high idle. Once warmed up it's wanting to idle at 1400 rpm today.

So what's next, sticky distributor?
_________________
1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6

fiskolja · Joined: 08 Aug 2024 Posts: 21 Location: Sweden

Have you done a leakdown test? Might be a vacuum leak

Also this thread might help
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924 NA 1977

Legitimate Salvage · Joined: 19 Dec 2023 Posts: 53 Location: NC, USA

I'd have to get the tools to do a leakdown test. But I feel like I'd have other problems if the cylinders weren't sealing properly. But I'm game to try it (especially if I get to buy a new tool).

The thread referenced was one of the first threads I found that seemed to describe my exact problem. Based on that, I replaced the vacuum hose that goes from the manifold to the brake booster. I also confirmed the function of the inline check valve. Not sure it made any difference in my high idle, but the hoses were shot and needed replacing anyway.

I've cleaned and adjusted my AAV months ago. It seems to be working properly. I would think if it were sticking open that nothing I could do would lower the idle... yet I can, with foot on the brake, let out the clutch until the engine bogs and it will (most of the time) return to 1000 rpm.

Which is why I keep coming back to the ignition distributor. It is a direct mechanical link between vacuum and idle. FWIW my dizzy has two vacuum connections at the vacuum pod. One is the traditional advance (on the front of the pod) and the other is on the back and (i assume) is for retard. The timing advance line goes straight to the intake manifold at the throttle body. The other connection ties into the vacuum lines that go to the deceleration valve and the brake booster line. Could the vacuum pod be faulty?

I suppose I can't rule out the idle screw o-ring either. Does anyone know the size or part number for that? I'd like to replace it just to eliminate that variable.

This weekend I'm going to clean out the throttle body to see if that has any effect.
_________________
1979 924 NA - AKA the Mid-life Crisis
2012 Audi A3 - The Daily
1991 MB 300D
1984 MB 300D
1966 Mustang I6