Engine Drops RPM when Idling?

[moritz55]

Hi, I've had my 1973 240z now for 18months. I've finally got it fixed up the way I like it inside and out. It has a rebuilt L24 , Firenza Aluminum Flywheel, 1982-83 5spd Tranny with New Clutch/Master Cylinder and an R200 Rear-End. Car runs super and solid.

My Question for both Observations: Is this normal, if not what could it be?

1) With car idling once warmed up (I keep it at 900 RPM, 17BTC) and the Clutch Depressed to the floor - it idles rock solid. When I Release the Clutch and let it out while the car is in Neutral - the idle drops to ~750-800 RPM. It then creaps back up to 900RPM after 10-20sec is my estimate. It's a noticeable drop however and very slow creap back up to 900RPM.

2) With the car idling once warmed up (at 900 RPM, 17BTC) and then the electric fans kick on, the car drops to ~750-800 RPM. Once the fans turn off the RPM resumes back at ~900RPM.

Many thanks for insight...

[Zs-ondabrain]

as for #2. This is typical because the draw from the fan motor requires more of the alternator. This puts a strain on the alternator and makes it harder to turn, so the motor slows a bit due to the tension on the belt.

The fan also takes power away from the ignition system and may slow or decrease the power that the spark plugs normally see at an idle. hense the slowing of the motor. New cars have crank and voltage sensors that tell the computer to speed up to make up for the draw on the system. So when the A/C or the fan kicks on, the idle will drop for a second then come back up because the computer tells it to.

and by the way, a 200 RPM drop, when the fan kicks on or clutch is activated, is nothing to be worried about. But if the motors idle drops so much as to die or not be able to recover, then you have a problem.

JMO

Dave.

[moritz55]

Many thanks Dave for the answers!

I did upgrade to the 60amp '82-'83 alternator with builtin regulator along with adding your ZXP conversion plug to replace the 240z regulator function. It's been super and rock solid electrically ever since. It never occurred to me that it was additional load on the engine from the alternator kicking in to support the higher amperage draw from the electric fans. Great news & Thanks !!!

I was wondering if for observation #1, if possibly releasing the clutch causes the overall Vacumm level in the engine to change slightly thus causing a slight change in RPM (~200 RPM drop). I did experience on my older 240z when I had a brake booster leak and could actually hear it hiss that when I pressed the brake pedal the vacumm leak caused the engine RPM to drop 200-300RPM when it was Cold; just after starting (Engine is Running Richer). I also did not have installed the Brake Vacumm Checkvalve inline between the Booster and the Engine. Then once the car warmed up fully and I depressed the brake, the engine Increased 200-300 RPM, with the slight change in Vacumm.

Thoughts and comments are welcome, and thanks again Dave!

Many thanks again ,

[Arne]

Re: #1 - Probably gear drag. The difference would be most pronounced when the transmission is cold, and less noticeable as the gear oil warms. What gear oil are you using?

[sblake01]

Also, remember he says he has an aluminum flywheel. That will lose its inertia faster than a stock flywheel.

[moritz55]

Thanks Arne, this is great insight.

I don't know what gear oil is in the transmission as it was completely rebuilt when I purchased the car. I know in my earlier 240z when I had the 5sp pulled and rebuilt, the actual gear oil used surprised me - I think was a standard SAE 10W-40W.

I'll watch to see if the RPM drop is less after it warms up.... as the gear oil warms up. Lord knows in Minnesota, we have cold weather and just had snow last week. This past weekend it was a balmly 40F-45F and dry so I had a chance to cruise in the 240z.

Many thanks,

[Arne]

Probably has 90 wt. GL4 in it, if they followed the factory recommendations. Pretty thick when it's cold. If this is the case, for your climate especially I would consider going with a good synthetic. I'm using Redline MT-90 in mine.

[moritz55]

Great advice, I'll be relocating to New York in several months. The weather will be warmer, however we'll still see 10F-30F days with clear dry roads for taking the car out.

Shall I assume the Redline MT-90 is good for summers as well as the cold?

Many thanks,

[sblake01]

I have it in all three of my Datsuns and have for years. I live in So Cal and I've had no problems.

[ozconnection]

From a slightly different angle, check your idle mixtures. I use an innovate air/fuel ratio thingy and whilst the engine will idle with a reasonably wide range of mixture strengths, the added load of an alternator doing more work or the release of the clutch pedal will load up the engine and cause the drop in rpm you're experiencing.

If your mixtures were right without the load, they probably won't be after it. Check your mixtures when the load is on the engine. Adjust accordingly.

I tune my car in gear (automatic) so I can sit all day at the lights with a mixture strength I know works and helps to stabilize my idle rpms. As a ball park figure, I run 13.0:1 with a warm engine, in drive and at 750 rpm (850 in neutral...thats only 100 rpm drop through an automatic trans!!). I also make slight carburettor adjustments for the anticipated weather, usually twice a year.

Cheers mate.

[moritz55]

Good idea, thanks for the insight. I did readjust the idle yesterday. Since I have a 4Bl Holley carb installed it's pretty easy to do.

I made the Carb mixtures for both ~1/8th Turn Richer, and tweaked the idle back up to ~900 RPM. Vacumm stayed pretty close to it's peak, maybe a touch drop , but I think this was due to the RPM slight drop. When the Fan kicked in it seemed to drop a little less as I observed on my digital RPM meter. It may be my imagination, but I'll continue to tweak it to minimize the RPM drops with the alternator load since my car is a 5sp manual and can't do it as convienently as you with the automatic transmission.

Many thanks,

[moritz55]

Well I made one last adjustment to the Carb Mixture valves and I'm pleased now. I ended up backing off on the mixture screws ~1/4 turn each making them Richer (I have a 4bl 390cfm Holley). I then had to increase the RPM up slightly back to the ~900RPM. When the Electronic Fans now turned on and kicked the Alternator ON, the load from the alternator only Dropped the idle to ~830RPM, less than 100RPM drop. I'm please with this setup.

Many thanks for the advice!