I'm not sure what's going on here, but I'm hoping somebody here will.

I've been driving my car ('74 2.0 all original engine and FI) for a few years now, I did some tuning last summer, and it's been running great since. But, having it out only once a week at most, this winter the battery had started to show signs of wearing out - the car was getting hard to start and often needed charging - so last week I bought a new battery and installed it.

Initially, this seemed to work great: I turned the key, the car started immediately, and it idled perfectly at 1200rpm. However, as soon as I touched the gas, the engine revved to 3000 and stayed there for about 20 seconds. Upon experimentation, it seemed that the engine would retain both power and speed of any throttle position for a ~20s period after the throttle was returned to idle. I was able to alleviate this to some extent by turning the idle-mixture adjustment to maximum and opening the idle bypass by 5 turns, but it still acts weird.

When I set the idle mixture to do anything besides flood the rpm down, the engine stays at 1500 rpm even with the idle bypass closed all the way and the AAV closed off.

My instinct is that it's a vacuum leak, but there's not a whistle to be hard or lose hose to be seen. Could it be (I hope not) something to do with the FI system?

Have you checked to see if throttle cable/linkage operates freely? It has been sitting...

Also - I always verify charging voltage w digital volt meter (use a better-than-harbor-freight one, I've found Harbor Freight meter's grossly inaccurate and will not buy meter's from them again) before battery changes.

Check the throttle linkage is moving freely. Also check the primary ground coming off of the battery to the chassis. Make sure that ground is good. I'd be checking and making sure all other wires connecting to FI components are well connected, which I am sure you probably have done already.

Which terminal did you disconnect first when replacing the battery? And when installing the battery?

Throttle seems nice and free. Clicks back to idle as soon as you let go of the lever, but the engine keeps going.

I am suspicious of the alternator; what should the voltage across the battery be when running?

And i always disconnect positive first. Ground strap seems good and secure.

From High School shop 55 years ago. .

Always disconnect the ground first and connect the ground last. .

The reason being if you disconnect the hot / positive and your wrench accidentally touches the body of the car or other metal you will have a giant arc flash and probably burn yourself. . same goes for having on a metal ring on a finger, potential of a shock / burn also. . with the ground disconnected first, touching the positive terminal can make a complete circuit because there is no path for the negative terminal, {unless you drop a wrench across the terminals]

Agree that what you describe sounds like a vacuum leak. But more likely something is binding.

Alternator has nothing to do with the engine revs, check the return spring on the cable (at the engine end) and as mentioned previously the pedal linkage and cable for sticking.

I'll definitely investigate the vacuum leak angle further when I look at in next, but I'm not even sure where to look. The only sound of whistling air I hear is the idle bypass.



Sticky throttle was my immediate thought, but as I said, it's entirely free and snappy. The problem persists whether I hand actuate the throttle or press the gas.

As for the alternator, I was wondering whether an alternator or voltage regulator malfunction could mess with or damage the ECU? Both components are original, but tested fine back in 2012 - which is why I wondered.

First I would put all adjustment that you touched back to where you found them. Then, Perform a charging system circuit test. (test battery and alternator) That new battery that you put in may not be GOOD. If all you did was change the battery, and now there is a problem... (i'm assuming all you did was change the battery) then the battery is likely the problem. Bad battery can cause all kinds of problems.

Disconnecting the positive terminal first is dangerous.

Make sure your battery ground and tranny ground strap are clean.
Throttle cable is a back route for grounding the engine, and it's not a good one.

Possibly the butterfly valve in the throttle body is sticking.

Changed your battery and this happened?

I bet your ground connection is bad and the engine is seeking earth through your throttle cable in the tunnel. micro welding the cable to the tube in the tunnel.

Try to completely disconnect the ground cable from the car and clean the stud on the firewall and reconnect.

" free and snappy" can be an issue , if the cable is too free, it could be touching the clutch cable on the side of the pedal . I had that issue last week after replacing my transmission , it needs to be free but just enough to hold itself from bending and touch the clutch cable.

I has been quite some time since I have been on this web site, but it is just like riding a bike... one never forgets and it is always fun. Reading about your problem, I have a suggestion for something you may try. Perhaps changing your battery has slightly affected voltage throughout the FI system and thus your throttle position sensor is now acting up. I think generally a worn/dirty throttle position sensor causes hesitation, but perhaps it can cause a lag as would be the case with a worn dimmer switch on a lamp.

It is an easy thing to remove, clean and re-install... I eventually replaced mine with a brand new one before they became a bit pricey. In my case throttle response was noticeably better with the new unit..God luck with your problem...

The D-Jet system is the flower of late 1960s technology. It has really big transistors in it, not integrated circuits. The voltage to the "ECU" ( I use that term loosely) can vary a lot and it will have absolutely no effect on the running of the engine. The thing is so over engineered it could probably survive a nuclear EMP pulse and still work.

I suggest that you clean the grounds and make sure they are intact.

FYI... the charging voltage at the battery should be around 13.5 volts.

Flower of technology......awesome.

Well, it seems everyone was at least a little bit right!

Unless you said vacuum leak. Then you were wrong. Because my work is airtight!

It seems that the battery ground was less than amazing, the throttle return spring was getting old, and the TPS was a little further over counterclockwise than was probably ideal. This combined into a state where the throttle would not quite return to idle, even when hand actuated, unless it was manually pushed back into position.

With all those thing remedied, and together with the rest of my confused precision tuning, the car now runs amazingly well.

Congratulations!!