Tachometer needle jumps sometimes - cause?

[82DMC12]

David,

I could try running my smoke machine while the engine is hot, that should reveal any heat-related vacuum leaks, don't you think?

Regarding the decel springs, you can see the photo of the springs above. They are not collapsed. Is there any way to test the decel spring theory without replacing the throttle body?

I did just try a spare idle ECU that painterdave had - no change, it's not the ECU itself. Could it be the idle motor?

I am going to put a timing light on all six plug wires one at a time and see if I can find if any of the cylinders is crapping out (for the tach issue)

[Rich]

Besides our batteries being the group 78 for physical size and the side post terminals, is there a rating in cranking amps that's right? I see some batteries have more cranking amps, but wasn't sure if more is always better in things like this, like maybe there is a limit where that's too much.

I'm unaware of a factory spec for a DeLorean battery aside from the Group 78 size. (Group 78 batteries are all side terminal type)
As mentioned, a smaller side-terminal battery can work.

Higher cold cranking amps (CCA) or cranking amps (CA) are better, mainly in terms of cranking/starting power as the term implies. Higher ratings typically come with a longer battery warranty.

No limit. A higher rating won't hurt anything besides your wallet.

[David T]

David,

I could try running my smoke machine while the engine is hot, that should reveal any heat-related vacuum leaks, don't you think?

Regarding the decel springs, you can see the photo of the springs above. They are not collapsed. Is there any way to test the decel spring theory without replacing the throttle body?

I did just try a spare idle ECU that painterdave had - no change, it's not the ECU itself. Could it be the idle motor?

I am going to put a timing light on all six plug wires one at a time and see if I can find if any of the cylinders is crapping out (for the tach issue)

I was thinking more along the lines of a system that changes as the engine warms up. You could try running the motor in the dark and looking for any sparks or blue glow that would indicate a problem in the secondary ignition system. A system wide problem would not single out one cylinder. A misfire on one cylinder would give you the jumpy tach and the bad idle. I think you are looking for two separate problems, one causing the fast idle and one causing the bad idle/jumpy tach.

[Jonathan]

I'm unaware of a factory spec for a DeLorean battery aside from the Group 78 size. (Group 78 batteries are all side terminal type)
As mentioned, a smaller side-terminal battery can work.

Higher cold cranking amps (CCA) or cranking amps (CA) are better, mainly in terms of cranking/starting power as the term implies. Higher ratings typically come with a longer battery warranty.

No limit. A higher rating won't hurt anything besides your wallet.

Thanks for the explanation. The extra warranty/extra cost sounds a little like how they price power surge bars for home theatre systems. More expensive version = longer/higher warranty.

Question about battery disconnect switches... could a wonky disconnect switch in some way cause irregular voltages and the like? I'm just thinking out loud as to what other things might affect things intermittently that haven't already been discussed.

[82DMC12]

Hey guys, I checked a few more things today.

1) Cleaned with a wire brush and electronics cleaner all of the bulkhead connectors since white/slate and some of the wiring for the idle parts go through these connectors.
2) Cleaned Fuse 1
3) Tested the thermistor at the Idle ECU (black/yellow and black/slate on the inner white plug, disconnected from the ECU) while the engine was still cold. On the 20K scale I get about 8.00.
4) Started the car and let it idle up until hot. After a certain point the idle get a bit rough and then slowly ramps up to around 1,200 RPM just as it had been before. As it climbs up, the tach starts to freak out and shown in my previous video.
5) While this is taking place, I hook up my Actron external tach to monitor directly off of the white/slate on the coil. The Actron is not bouncing all over like the dash tach does.
6) While the car is idling, I hook up my timing light and check each cylinder while the tach is freaking out to see if there is any change to the flashing. There is not. All six plug wires seem to be firing normally with no drop out or monkey business
7) Unplugged the inner plug on the idle ECU and the RPM's jump up to about 2,000 RPM as expected. The tach is still bouncing around so this tells me the tach bounce is not related to the Idle ECU.
Shut off the car now that it's hot (fans on) and checked the thermistor again at the Idle ECU on the same pins. Now my 20K scale is reading only 0.86.

Very interesting because I'm reading some diagnostics information online that suggest the thermistor should read 8K to 12K when cold and 3K to 5K when hot. Clearly I'm not interested in replacing the thermistor unless I am positive my readings are bad and they mean the thermistor is dead/dying.

Wondering if anyone can confirm and even better if someone whose car is running correctly would repeat the same thermistor test?

Would a thermistor reading these values result in a randomly high hot idle? Perhaps the tach is still a different issue but maybe the thermistor explains the idle.

[Bitsyncmaster]

I have been helping another owner with his idle problem and he got about the same resistance as you on the idle thermistor. But mine at 75 deg. F (cold) is 20.7 Kohm and 1.12 Kohm when my fans turned on and I shut the engine off.

I would also like to see what some other owners are reading on their thermistor. I used my reading when I designed my ECU software but I don't think that difference makes much change.

[82DMC12]

I have been helping another owner with his idle problem and he got about the same resistance as you on the idle thermistor. But mine at 75 deg. F (cold) is 20.7 Kohm and 1.12 Kohm when my fans turned on and I shut the engine off.

I would also like to see what some other owners are reading on their thermistor. I used my reading when I designed my ECU software but I don't think that difference makes much change.

Thanks Dave. I appreciate the measurements. I'd be forever grateful if a couple more owners would do the same test and report back!

[Jonathan]

Andy, I'm not sure if you're still chasing this fluttering tach issue, but thought I'd share some recent progress with the same issue on my own car.

About a week ago I got to reinstalling the alternator belt, one-handed tensioner bracket. I replaced my alternator a couple seasons ago and forgot the part where you had to drill out slightly the one attachment hole to get the securing bolt to go in completely. I had everything all back together when I realized this and so just put the original back in instead.

The alternator had to come out completely to get the proper angle on drilling out that hole. That part was easy and when I reinstalled everything, I put the new tensioner bracket on as well as a new belt. Now, while I don't think it was related to the previous belt being too slack, that work has eliminated the fluttering tach needle.

The step in there that I think did the trick was to really focus on making sure the brown cables were tight on the alternator post. I snugged up the post on the alternator first and then made sure the two main brown cable jumbles were aligned well enough to come together flush and snug. I got everything tightened back up and have over 100 miles on the car and the tach needle has not fluttered a single time.

I'll keep an eye on it still, but wanted to share in case all of that was just traced back to an alternator post electrical connection not being good enough.

[Azar]

Hmm well I'm actually replacing my alternator next week because it's making some noise, is probably 20 years old, and only making 12.4V when the engine is hot and fans and headlights are running.

Hi, I have same problem with the needle but very rarely in idle, mostly at 3000-3200 RPM.
Btw, my alternator is also delivering 12,5V with AC and Fans on and 14.0 with everything switched off.
Can you confirm that with the new alternator it is now improved? Thanks.

Just another observation: I think it got worse after the engine overhaul and I can see that checking the alternator cables should me on my list (after I fix the engine).
Interestingly, almost all of my instruments are now reading lower than before engine overhaul:
- Voltmeter is showing 1,5V less than actual (same as my aftermarket radio which has screen mode showing voltage)
- fans kick in on 190 instead of 210 before and switch off at 150 instead of 160 before.
- oil pressure showing less than of measured with the manual gauge
- fuel is showing 1/4 less than in reality

[82DMC12]

Andy, I'm not sure if you're still chasing this fluttering tach issue, but thought I'd share some recent progress with the same issue on my own car.

About a week ago I got to reinstalling the alternator belt, one-handed tensioner bracket. I replaced my alternator a couple seasons ago and forgot the part where you had to drill out slightly the one attachment hole to get the securing bolt to go in completely. I had everything all back together when I realized this and so just put the original back in instead.

The alternator had to come out completely to get the proper angle on drilling out that hole. That part was easy and when I reinstalled everything, I put the new tensioner bracket on as well as a new belt. Now, while I don't think it was related to the previous belt being too slack, that work has eliminated the fluttering tach needle.

The step in there that I think did the trick was to really focus on making sure the brown cables were tight on the alternator post. I snugged up the post on the alternator first and then made sure the two main brown cable jumbles were aligned well enough to come together flush and snug. I got everything tightened back up and have over 100 miles on the car and the tach needle has not fluttered a single time.

I'll keep an eye on it still, but wanted to share in case all of that was just traced back to an alternator post electrical connection not being good enough.

Interesting observations! I'm sure power problems can cause all sorts of whack stuff.

I see I forgot to update this thread. I had a couple threads going at once :-) Anyway, not sure if I mentioned it or not but a few months ago I had the distributor out so I could replace the o-ring on the shaft (while doing a valley dive). At that time I also replaced the impulse coil since I had an NOS one still in a bag, sealed up, that I bought like 15 years ago and never installed. Since I had the distributor out, I replaced it. After that valley dive is when my jumpy tach got so much worse. I ended up pulling the distributor out and putting the original impulse coil back in AND I replaced the black "tach fly lead" that snaps into the distributor (and then goes to the 2-pin Bosch connector in the firewall). It must have been one of those two parts causing my issue because my tach problem went away and hasn't come back since.

You mentioned the brown wires on the alternator. A few weeks ago I pulled them all out of the car and replaced them with 4 awg welding cable and new hydraulically-crimped copper terminals. I think that helped my voltage issues too.