75 914 with D-Jettronic, need help diagnosing stuttering Options

[Rant6]

Hi folks. I have a nearly all original 1975 914 with the 2.0 engine. The car has 93k miles on it, and I have owned it for almost two years. I have been troubleshooting one particular problem for quite some time. The car runs perfectly with one exception. The engine hesitates or stutters often. It is a hard stutter, that is, it actually feels like maybe the ignition is cutting out for a fraction of a second, and so it causes a bucking sensation when driving the car. There are few discernible circumstances where this always occurs or never occurs. It seems to be random, and it can occur under easy acceleration, steady throttle, or off throttle (usually VERY slightly off throttle). It can occur under cold or hot conditions. It happens every time I drive the car, sometimes infrequently, but more often it happens constantly. It seems to happen most often in the middle of the RPM range, that is from about 2500 to 3200 RPM.
Some of the emission control stuff has been disconnected or removed from the car including the charcoal canister, the distributor vacuum retard, (I don't think there was a vacuum advance because I can't find where the hose would have connected to), the vacuum line to the decel valve, and an electrical 2-prong plug that goes into the intake plenum. I have done the following things:

Fuel injector cleaner
New plug wires
New plugs
New rotor/cap/points
New air cleaner
Re-attached the electrical plug in the intake plenum (this caused the idle to surge up and down 1200 RPM, so I disconnected it - what is this thing?)
Re-attached the vacuum line to the decel valve
Replaced the circuit board in the throttle position sensor, and aligned.

I really thought the last item was going to be the fix, but it had no effect at all.
Can anyone offer some possible causes?

Thanks,
Mike

[76-914]

Did you carefully examine the fingers on the TPS when you replaced the board? If they are twisted a tad they ride off track or on their sides. Or, just disconnect the plug at the TPS to see if it goes away. Also, what is your fuel pressure? I've read that bumping it up a pound will sometimes eliminate this bucking but I can't confirm it.

[TheCabinetmaker]

I agree. tps!

[Rant6]

I did look at the fingers on the TPS, and one of them does ride very close to the edge of its associated strip. I'll take a closer look to see if it is losing connection. If I disconnect the TPS, will the car still run?

Thanks,
Mike

[Java2570]

That plug on the intake sounds like the Air Intake Temp sensor....if your idle is oscillating when it's plugged in, it could be shorted out causing a lean condition. When it's unplugged, it makes the car run a little richer.
If your TPS is good, I'd be looking at a lean fuel problem at part load....maybe check out your fuel filter to make sure it's not clogged or a slightly kinked fuel line. It's a good idea to know what your fuel pressure is and how much flow you get out of your injectors also.
This is a great site: http://members.rennlist.com/pbanders/djetparts.htm

[jasons]

When I had this problem I believe it was new (currently NLA) FI points in the dizzy that finally solved it. The contacts will be OK, but the dogs that ride the cam on the dizzy shaft and control the open/close of the points get worn and the points don't open/close as well as they should. As a result you may loose one or both banks of injectors.

Yes the car will run without the TPS connected. But it may run poorly.

[dangrouche]

trigger points; when I changed mine on my Djet, it was like night and day, combined Pertronix with new trigger points and the car starts like an electronic ignition car; standing outside, twist the key and it starts right up and idles solidly

[JStroud]

Another posability is the MPS, had a problem with mine, car ran fine until the first time I had a major elevation change, car was cutting out, unplugging TPS did nothing, finally talked to a 914 shop in Albuquerque , where I was, he had me tap the MPS with a screwdriver handle while the car was running, idle immediately leveled out and car ran great the rest of my visit, however then my MPS was stuck at that adjustment, when I drove back home got back to sea level, car started cutting out again, tapping didn't help this time.

I ordered a rebuilt one from Jeff Bowlsby, haven't got the engine back in yet to confirm the results, but maybe something to check out.

Jeff

[TheCabinetmaker]

trigger points; when I changed mine on my Djet, it was like night and day, combined Pertronix with new trigger points and the car starts like an electronic ignition car; standing outside, twist the key and it starts right up and idles solidly

thats exactly how the car is susposed to start. With or without points.

The car will actually run pretty good with the tps unpluged. If it runs better unpluged, the tps is the problem. I would reset the tps again.

There is also a problem with the replacement boards. I think it is more an installation problem than a board problem though.

[gunny]

I had the same type of problem at about 3700 RPMs. Your sounds different but...

Buy a wideband Air/fuel sensor and gage. It will tell you a lot. My problem was that it was running too lean at that RPM and most of the time. This lead me to adjusting my MPS after I made sure my MPS was holding a vacuum and I also checked the calibration. I found that because I have a 76 cal with all of the EPA stuff removed I need more fuel than the stock MPS was calibrated for. Even if this isn't your problem I would say that the gage and sensor were the best investment I made in trouble shooting the tuning of my car.
Before I bought the A/F meter I purchased a spare ECU, Distributor, MPS, TPS, and Trigger points after completely replacing vacuum lines, plugs and adjusting valves and ohming every wire.

Check the MPS and buy an A/F setup.

Good luck

[r_towle]

There is an idle circuit that only activates in a single position.
The TPS is designed for many cars and many throttle bodies.
There is ONE way to mount it...its really not adjustable once you mount it correctly.
The pic is as mounted with the throttle body closed.
You can pull the throttle body with the TPS attached and set it up on the bench...no meter needed.
Use the picture below.

[WPEngels]

Hi,

I had the same symptoms on my '74 car after swapping the engine. It ran absolutely fine before, but stuttered after putting the engine back in.

I considered replacing several parts but luckily before spending a lot on new parts, I found a loose connector on the common ground point on the engine (on top of the engine, just in front of the transmission). Also, one of the cables was almost compleely severed. Must have been me trying to get the engine tin back into place, catching the cable between the tin and the engine casing.

Anyway, repairing this cable and making the connector fit tightly again solved the problem.

Kind regards,

Wilbert

[Rant6]

That plug on the intake sounds like the Air Intake Temp sensor....if your idle is oscillating when it's plugged in, it could be shorted out causing a lean condition.
This is a great site: http://members.rennlist.com/pbanders/djetparts.htm

Thanks for the site link. It is indeed a great site. Here is a pic of the electrical plug on the plenum that is disconnected. If this is the Air Intake Temp Sensor, are these things available?

....and yes, I'm embarrassed that my engine is worn looking, but it is 38 years old!

Thanks,
Mike

[r_towle]

look up Jeff Bowlsby.
He manufactures new Fuel Injection Wiring harnesses.

Rich

[Java2570]

In reference to your engine photo, trust me we've all seen much worse. That is the air intake temp sensor and I'm sure someone will have a good one if you post a WTB ad in classifieds here. I'm guessing that it's an issue in your wiring that's causing your problems. The wires and connectors all get pretty baked after all those years and Jeff Bowlsby makes great replacement harnesses for our cars. I believe the Anders
link has the procedure for testing that sensor to determine whether it's ok or not so you may want to do
that before buying another. For wiring, a quick thing to do is to check and clean all your connections and grounds and see if that improves anything.