Chasing a Ghost in the Machine, It’s been a year and I thought I had it. Options

[mgphoto]

About a year ago my usually good running engine began to stumble, slight backfire through the exhaust, between 1000 to 3000 rpm. I first suspected an electrical fault, as I would shake wires in the engine compartment and the issue would be gone.
This happened several times with different wires. I decided to tin the ends of the wires attached to the positive battery terminal. This seemed to work, for a while.
I began testing other wires for the D-Jet, I had success with reattaching the TPS connector, for about a day.
I began following a post about a member who had trouble with his 123 dizzy, his issue was related to the D-Jet pulse producing erratic fuel delivery.
OK, spoke with the tech person at 123 in Europe, we could test it using an oscilloscope.
I have a small pocket unit that I can test it with. This past Sunday I decided would be a good time do the test.
My plan was to take the car out for a warm up drive, once around a freeway loop gets my oil temp needle to move.
After exiting the freeway, I stopped at a traffic light and the car stalled, I’m used to that happening sometimes as I have a lightened flywheel, but this was different, like I turned the key off.
I turned the key to restart the engine but it sounded like a rattling chain, I never heard that sound before. I tried the starter again, now it had the typical hot start problem, after a moment I got it to run but the idle was now at 3400 rpm.
Since I could control it I drove it home and began checking simple things like throttle cable, TPS loose, MPS held some vacuum using the suck test.
I’m going to take a closer look at the throttle body.
Anyone have their D-Jet idle jump to 3200 rpm and what was the reason?
Thanks,
Mike

[brant]

Return spring maybe

[76-914]

spot weld failure on the clutch cable tube? (IMG:style_emoticons/default/beerchug.gif)

[Dave_Darling]

A 3200 RPM idle is an air leak. A *BIG* one. Check for hoses that have come off the manifold, then check for hoses that have come off other stuff in the bay. Look down the throttle bore to see if the throttle plate is jammed open or is damaged. Then check for stuff like holes in the plenum or a split plenum.

There's a reasonable chance that you have multiple problems adding together...

--DD

[JeffBowlsby]

The MPS mouth suck test is completely irrelevant, the mouth can only generate about 10% of the vacuum pressure an MPS sees/needs.

Use a hand vacuum pump and check it with a gauge.

[pbanders]

Good advice from all. I've found that finding the cause of such issues by chasing the symptom is often unproductive. What's worked for me is to go through everything in the D-Jet system, the intake system, the ignition system, and the charging system. My page has diagnostics for the D-jet components, and there are other resources out there for doing the same with the ignition and charging systems. Mechanical and vacuum inspection of the physical components (e.g. throttle plate, linkage, hoses, proper routing, etc.). Only when you are sure all of this is in order, do you start chasing symptoms. One of my old mechanics (Bob Grigsby) told me that when he had customers come to him about a running or driveability issue, he'd listen to them, take down what they said. Then he'd go and do a series of inspections, adjustments, and procedures he did on every single car that came it. He said after he'd completed this process, almost every time the problem was fixed. Only then would he chase the symptoms.

[mgphoto]

Big air leak!
Major clue, going to check it in a few days.
Thanks.

[mgphoto]

Ok, I’ve gotten this far.
I removed the air filter, 2.0L exposing the throttle body, turned the air screw in all the way, idle went from 2500 to 1500. Next I placed my hand over the throttle body opening and the idle fell to 900, it didn’t stall so there is definitely an air leak.
I didn’t see any problems with the manifolds so I pulled the throttle body and the shaft was very stiff.
Next I’ll inspect the hoses and the plenum.

Thanks for pointing me in the right direction.

[914sgofast2]

I would suspect your AAR valve has gone bad and needs either a very thorough cleaning or the heating element replaced. When the AAR valve goes bad from either cause, the valve sticks open and results of lots of "fugitive air" entering into the intake manifold downstream of the throttle body, resulting in a high idle rpm in the range you are experiencing with your 914.

[emerygt350]

that t that the pcv goes to on a 73 is often the culprit for a leak that big, not sure what year you have. There are only so many spots that can let in enough air to keep your car running with your hand over the TB. The other would be the AAR.

[bkrantz]

Is starter spray (ether) still a thing? Spray this at different connections and when the engine revs up you found the leak.

[76-914]

Disconnect everything from the vacuum side except the MPS. Then see if it improves. (IMG:style_emoticons/default/beerchug.gif)

[mgphoto]

Don’t see the AAR as the issue as it hasn’t worked for 30+ years and it started working after the engine stalled and opened although it was powered for 20+ minutes.

[emerygt350]

Don’t see the AAR as the issue as it hasn’t worked for 30+ years and it started working after the engine stalled and opened although it was powered for 20+ minutes.

I'm just saying there are only so many spots a leak that big can exist, the hose leading to the AAR is one of them. It didn't get knocked off did it? The bit heading to the cold start valve.

[mgphoto]

Don’t see the AAR as the issue as it hasn’t worked for 30+ years and it started working after the engine stalled and opened although it was powered for 20+ minutes.

I'm just saying there are only so many spots a leak that big can exist, the hose leading to the AAR is one of them. It didn't get knocked off did it? The bit heading to the cold start valve.

All seems connected correctly, I will start removing the hoses attached to the plenum. I need to start cleaning the throttle body.

[rjames]

Check the Plenum too. They rust out from the inside.
If it hasn’t already been suggested and you haven’t re-tried it yet, do a smoke test to see where the leak is.

[jim_hoyland]

I had an air leak caused from a faulty 3-way connector. The connectors had deteriorated and air was entering and hose that connected to the plenum.

[mgphoto]

Ok, I pulled the hoses on the plenum, there is a rubber connector that has a split and one of the other hoses looked to be split. I ordered replacements but I think I’m still going to pull the plenum, and look for cracks.

[rjames]

Ok, I pulled the hoses on the plenum, there is a rubber connector that has a split and one of the other hoses looked to be split. I ordered replacements but I think I’m still going to pull the plenum, and look for cracks.

Sounds like you found your leak!