Hi.
My 200HP Vert FC is sometimes is having a weird engine stumble. If I had to describe it, the engine completely looses power, then power comes back and the car jumps forward then repeats. However, that only happens, when the gas pedal is only slightly pressed. If I step on it it accelrates fine. If I drive fast enough, that I have to keep the gas pedal pressed, it runs fine. Weirdly it even idles fine. But keeping a steady speed in town in 4th gear? Not possible.

At first I thought the problem only happened while it rained, but last week I had it on a completly dry day while driving through a tunnel. And at the end of the tunnel when I cut my headlights, the stumbling instantly stopped. So the fact, that I drive with whipers and headlights in the rain caused the issue, not the wetness itself.
It is hard to recreate it, it's not only driving with headlights or something like this, but every time the stumbeling happens, it clears up after disabling the headlights.

So, now what could it be?

- battery? Is pretty old, but starts every day without any issue.
- generator? Brings around ~13,5V after starting, going to around ~ 12.7V when battery is full. But, it loses ~0.2V when enabling headlights. (Measured with my internal instrument, not a digital meter.)
- sparks? have around 5k kilometers on it and look ok.

Anything I can try or measure to help finding this fault? It's annoying

 

Take all I’m saying with a grain of salt because I’m not very experienced with electrical gremlins, but I would make sure your alternator is good by checking voltage and making sure its supplying enough voltage. IIRC normal is 13.5-14.5 at idle

My thought process is that if the alternator isn’t making enough juice under low load it could cause headlights/wipers to siphon current from the ignition system or something but when at higher rpm it generates enough voltage to power everything.

Another thing that might fix this would be to get LED headlights that draw less current than the halogen ones. It wouldn’t fix the root of the problem if its the alternator but it may fix the symptoms.

 

Measure voltage between the alternator's housing and battery negative, and between the alt's positive post and the battery positive (both with and without headlights). Should be close to 0 volts in either case. The 12.7 at idle is rather low.

 

12.7 is low, but you should verify with a multimeter as others have suggested. The internal gauge is not very accurate, especially after 30 years.

I did find on my car that the engine runs a bit worse without the alternator belt on. I think the ignition system really wants full voltage to provide good spark. It's anecdotal, but it's a good first place to start.

 

Thanks for the tips. I will measure the voltages at the weekend.

 

Sounds like grounds to me. I agree with Barn, but I think he means check resistance between the alternators housing and battery negative. Should be less than .2 ohms. Do the same for other ground points and inspect all ground points. Lots of threads on this forum about grounds.

 

You can measure the resistance but without a mili-ohm capable meter its fairly useless for such a high amperage connection. If you have 1+ ohms in those lines the car will never charge the battery . 1 ohm = 12 amp at 12 volt. Meaning you get ~14 amps at most, ignoring any other electrical load (resistance) in the system. Stock alt is 60 amps. You are getting basically no power through the system. And that is 1 ohm, which is about the limit of normal multimeters.
If you have a mili-ohm capable meter (I do, best investment ever if you want to seriously inspect connections) you can measure almost any ground, cable, connector, switch etc. The other solution is to measure the voltage drop over the cables, which is what I recommended. If there is resistance there, there will be voltage. This directly correlates to real problems (50 mOhm is a real problem here, while for a switch that controls a relay that is a non-issue), if there is voltage drop over the cable that is a problem, always. Of course, as you increase your load there will be more voltage drop in these lines, so do the test with max expected load as well.

Of course measuring voltage between battery posts and alternator + and alternator housing is kindof the same as well, if they are not equal the loss will be in the cables, somewhere.

 

Ok my guy. I'm not going to argue with you. Go get a meter and check continuity between a ground point and the negative battery terminal on your car. Be sure to report back the results.

This is an 80s Japanese car, not a SpaceX rocket ship.

 

Battery-body is ~0.4 ohm with normal MM, which is about as low as it goes or ~1 milli-Ohm with the proper instrument, which is not as low as it goes (but close). In other words, fine. Alternator to battery is ~50 milli-Ohms which is less than stellar, coincidentally its also 0.4 ohms with the normal MM. Better MMs can sometimes go down into the 0.1 ohms range acurately, but in a situation like this where voltage is easy to measure it is a much more robust way in my opinion.

For more expensive stuff like planes, you'd measure the grounding between panels with instruments that cost, well, lets say considerably more than the 100$ that the cheapest Kelvin measurement capable MM costs. Is this needed for a car? No not really. Mostly because for high current circuits its usually easy to measure voltage drop, and low current circuits won't have problems with sub-1-ohm resistances for which you'd need this. Is it good to have? I think it is, absolutely. Makes life easier. Whether its worth it for you is not for me to decide tho.

Edit: The 50 m-ohm was about half volt drop if I remember correctly, think with the headlights off? Not sure to be honest. The car still charges like that, barely.

 

Okay, did some measurements, hope I did them correctly.

First, Battery was at 12,6V after sitting for a week. Resistance from negative terminal to the alternator housing was 0,2Ohm, which is propably the lowest possible measurment. terminal to chassis was 0,3Ohm. Positive terminal to alternator plus was 0,2Ohm as well.

Started the car, Voltage jumped to 16V then fell fast down to 14,7V. 14.5V with headlights on. All measured at the battery.

Waited a minute then did the reference measurements:
Alternator voltage (alternator plus to alternator housing) 14,2V with and without headlines. Voltage between alternator positive to battery terminal was 0,05V. Voltage between alternator housing and battery terminal was 0,05V as well. At the battery it was 14,2V without headlights and 14V with headlights on.

I let it run for 10min and Voltage at the battery dropped to 13,8V with headlights and 13,9V without. Stopped the engine and battery voltage showed 13,6V.

 

That looks... fine? All above the battery voltage so the alt is supplying the power. I assume the drop after 10 minutes is from the idle dropping down. Does the internal voltage decrease when the problem appears?

 

Not noticeable on the internal instruments.

To my knowledge the voltage at the battery will drop with the battery getting full due to increased resistances. I will try to get longer measurement, to see where it ends up.

 

Theres a ground for the front harness near the right headlight. Inspect it and all other main ground points including the ECU ground.

 

Can you tell me where that is exactly. Is it below the Air Filter? Haven't found it looking around the headlight.

I have inspected the ones at the ECU, at the top of the engine and by the ignition coils. They look fine, but I cleaned them regardless.

 

I cant really remember where it was, somewhere near there. Follow the front harness from the left headlight to the right until you find it. I think its under the front right headlamp (when theyre open) on the left side if you're sitting in the car.

ECU ground is on the top of the rear rotor housing.

 

I believe the stock location is under the airbox. That's where mine is, but I've also removed that part of the harness for cleaning before, so it could have moved when I put it back.

Check under the airbox, and also by the relays under the airbox as well.

 

Was on tour the whole day. No issues, until I had to drive with headlights on. Then it was stumbleing all over again.

Tried to get a video, but its hard while driving.

Video on Youtube

You can see it at the rev meter. The clicking sound is the drive shaft switching between accelrating and decelrating. (Now sure how to explain that better).


Tried voltage measuring afterwards, still looks ok.

 

Try unplugging your headlights behind the bulbs and see if you can replicate the problem.

if it still happens try unplugging the 2 gauge cluster connectors.

 

Thanks for the idea.

Car is now in the garage for the winter, and as I cannot replicate it without driving, I will have little updates till spring. But I will find and clean all those pesky ground connectors during maintenance.