Quick background: My '73 FI 914 2.0/4 engine seized up and I replaced it with another 2.0/4 engine that I had from my parts car that the body had rusted out. That engine was running fine when I pulled it about 10 years ago. I reused all the existing sensors on the top of the engine that died, that were working when it died. Just put in new dist. cap and rotor (had electronic ign. which I left in place), new spark plugs, replaced oil/filter, and adjusted the valves before putting in the engine. After getting the new (old) engine in, hooking it all up, except all air filter plumbing, the engine started right up.

Problem: After the engine idles for a few seconds, it revs up uncontrollably and I have to turn off the ignition to stop it. The throttle is closed and not even being moved. I'm at my wits end. I've tested everything I can think of and everything seems to be working. I've tried adjusting the timing (rotating distr., vacuum hose disconnected). This has an affect on the initial idle but it still revs up no matter how I adjust it. I can't actually do a proper timing adjustment since I can't get the engine to idle.

Does anyone have an idea of what may be wrong?

Thanks,

Clint

Vacuum leak possibly. Test with propane or carb cleaner when the engine is running.

Howdy Neighbor! I agree, it sounds like a vacuum leak. Have you replaced the vacuum lines?

OK, I'm not the techie. What vacuum lines? I have the air cleaner off as well as all of the hoses that attach to it. Could you be more specific?

Click to view attachment

My idle goes up, but takes a LOOONNNGGG time to come down
The AAR heater is shot. Rebuild the AAR and fix the heater by using 13 ohms worth of wirewound resistors to act as a heater.

My idle goes up, but NEVER comes down, and I've got the air bleed screw turned way in (or at the stop)
Air leak. Where? There are plenty of sources, here are a few just to get started:
AAR stuck open - this is VERY common - remove the AAR, invert it, and flush it with WD-40 through the port until it frees up
Cracked hoses (split and/or cracked at the ends and not snug on the ports)
Defective MPS (leaky)
Defective vacuum advance/retard cells on the distributor
Leaky CSV or throttle body gaskets on the plenum
Cracked plenum (air manifold box), or leaks through the body seams
Cracked intake runner boots (not snug on the plenum or intake pipes)
Leaky injector seals
Leaky intake manifold gasket at heads
Defective or missing PCV valve
Cracked rubber vacuum hose fittings
Leaky throttle body shaft bearing
Leaky CSV
Leaky decel valve
Leaky air temperature sensor
Check 'em all, check 'em again, and check 'em once more.
Timing overadvanced: Yes, I told you to check this, but I've seen this as the cause of high idle more than once. MUST BE set to factory specs using a timing light. Make certain your marks on the fan are correct!!!

Your engine creates a vacuum when it is running. Your engine should have vacuum lines routed all across it. If one of these hoses breaks, becomes brittle, or is disconnected somewhere, your engine could be sucking in extra air causing your engine to rev high. You should make sure all these lines are there, and have a tight connection and replaced if neccessary. Here is a diagram from HERE that shows where all of them should be.

Click to view attachment

Note: The lines to the top of the air cleaner assembly are not under vacuum so those won't be a problem if disconnected

Intake gaskets and carb mounting gaskets can also cause vacuum leaks. Intake gasket leaks are VERY common on these engines.

But would a gasket leak cause a runaway? I mean it revs up like the accel. is floored.

yes, a good vacum leak would

First of all

How high do the RPM's go?

I don't have a rev limiter on the rotor (I planned to buy one, but I got a maintenance kit that didn't include one). It quickly revs past red line in just a few seconds. I have to turn off ignition to stop it. It's like I had the accel. fully depressed (which I didn't).

I plugged the intake to the AAR in case it was the cause. That didn't have any affect other than the car was a little harder to start. The hoses going to the decel valve are tight and in good shape. So is the hose coming from the oil filler cap. The vacuum hose to dist. is tight and in good condition. Connecting or removing it does not seem to have any affect.

I guess the next step would be to take off the throttle body and remount it in case there is a leak there. Just doesn't seem likely that even if there was, it would be that big. If that doesn't work, I guess I can remove and check the intakes, their gaskets and the injectors (that's a real pain to do). Does that seem like a reasonable approach?

Thanks for all the help.

Clint

A quick check to see if you have a vacuum leak it to put your hand over the top of the throttle body and if the engine still runs, you have a vacuum leak. The air has to be coming from somewhere. What about the idle adjustment screw on the side of the throttle body? Is it turned out too much?

The idle screw is all the way in. I too was wondering how the engine could rev that high with the throttle body all the way closed, but I couldn't find any obvious problem with a visual inspection. I've got to go to some soccer games with the girls this morning, but I'll check all this out this afternoon when I get back.

look at the manifold intake sleeves at the throttle body, there are 4 of them,

I had a crack in my plenum around one of the support tubes. My idle would surge to 4000 rpm at start up. Replaced the plenum and no more problems.

Click to view attachment

Well, I took off the intakes to check the gaskets and that the injectors were properly seated. Of course, I didn't see any problems with them either. What I did observe while I was doing this is that the intakes to the plenum have lips on them (larger diameter) at the body (see photo). So when I reattached the intakes, I made sure that the sleeves were very snug to the body. Yep, that solved the problem. I'm not sure if that is what you meant detoxcowboy (I though you meant to check that they didn't have any cracks/leaks), but if it was, you were dead on.

Anyway, thank you everyone who has helped sort this problem out.

Regards,

Clint

Click to view attachment

PS: no jokes about the photo. I know it's really dirty, but it's the one that came off the engine that was sitting in a barn for 10 years!

It always pays to check the simple stuff...

glad it was a simple solution.

Kevin

Nice. Glad to hear you got it figured out!