Driveline drag at speed.

[Walter Moore]

Something strange happened with my Z today, and in 30 years of driving I have never experienced anything like it. I hope someone else has some idea, or I may have to rebuild the entire driveline.

I was driving around this afternoon, and suddenly the car would not go faster than some fixed speed, which seemed to be about 60MPH. I first noticed it in 3rd gear. I was accelerating briskly, and when it got to about 60 in 3rd the engine bogged down like it was in the wrong gear! That isn't the problem because it was somewhere around 3500 to 4000 RPM, and the engine pulls strong in that range.

I pulled off of the 4 lane road, and turned around and tried several other times. Each time the car just would not go faster than about 60. I found that if I tried to really push the acceleration it would pull all the way to 7000 RPM in first, and pull hard in 2nd, until it got to about 6500 then the engine just couldn't pull the load anymore.

No matter what gear the car was in, it just lugged the engine when it got to this particular speed. I had the same problem in 2nd, 3rd, and 4th.

I turned off the radio and listened for strange noises, but with the loud exhaust I couldn't hear anything unusual. There might have been a noise coming from the back end, but it wasn't clear where.

(All this was in a straight line. I wasn't turning when this happened.)

I decided to just drive slowly home, and after a few minutes the problem went away. I didn't notice exactly when it cleared up, but at some point I looked at the speedometer and realized that I was going 75MPH, and the engine sounded normal.

Any ideas from those of you who have actually had operational Z cars for a long time?

[d240zx2]

Ok, I'll share my ignorance with the rest of you....

It sounds like a partial fuel blockage or failing fuel pump. Here's why I think that.

1) The revs are limited when the engine is under more stress, e.g. 3000 rpm in 4th and accelerating v. 7000 rpm in 1st.

2) The problem diminished with time at speed, which allowed the diminished fuel flow to gradually produce the additional BTUs required to propel the car at higher speeds. I think the problem may still be there, albeit disguised in lower gears.

I would check fuel filter(s) and fuel pump output. Also check return lines for blockage.

Take two aspirin and call me in the morning.

Regards,

[Walter Moore]

Thanks for the suggestion, but I doubt that is the problem.

The entire fuel system from the tank to the carbs is new. Tank has been cleaned, and lined. All of the lines are new. The pump is new. The carbs are new. The hoses are new, and the problem came and went while the engine was running and hot.

Besides, I know what fuel starvation feels like, and this was not fuel starvation.

I am guessing that it is in the differential, or drive shaft. (Half shafts perhaps?). I recently resolved my differential thump problem by replacing the front differential mount, and tightening the nuts on the bar at the back. Could something in the differential been damaged by the thumping earlier?

[sblake01]

I had a transmission that was so bad that you could actually feel the drag but the car (my 810) would still top 60. It actually caused the car to run hotter than normal due to the extra stress on the drive train. When I removed it, I could move the input shaft about an inch from side to side. When I replaced the trans, all of those symptoms went away.

[Walter Moore]

My wife and I went for a drive this afternoon to experiment with different acceleration rates just to see what would happen. At first the problem wouldn't happen. But once we got everything up to temperature it came back.

For the record, it happens at a registered speed of 62 MPH. It doesn't seem to matter how hard the car is accelerating. The first time that it happened I was accelerating hard in 2nd. Once we were at that speed it would go no faster.

Then we drove through town at slower speeds and all seemed to be ok. So we took it onto the interstate highway to see if it would come back. We followed a really slow driver onto the highway, and just gently brought the car up to speed. But once again as soon as the car hit 62, that was all the faster that it would go. I pushed the accelerator pedal to the floor, and could hear the engine groan like it was trying to speed up, but the car just shuttered and kept going the same speed.

So I guess my car is going to speed the winter up on jack stands after all.

If it is the transmission, where is a good place to pick up another one?

[bigoak]

You can pick up later 5 speeds floating around wreckers or on these usual Z websites (there is an active thread on this right now). 4 speeds are just as common as many are swapping out 4's to put in 5's...

Minor search and there should be plenty of cheap alternatives available (<$100).

[Walter Moore]

I think that sblake01 may be right about the problem. (Thanks)

I took the drive shaft, half shafts, and differential out of the car and couldn't find any problems. Neither could my neighbor who has his own repair shop. I offered to pay him to rebuild the differential if it needed work, so when he said that it is in good condition I tend to believe him. (Who turns down money?)

So I went with the assumption that it was the transmission, and bought a used one out of an 83 ZX that my neighbor bought to restore, before he realized that there wasn't enough left of the body to bother with.

Tonight my son and I managed to remove the 4 speed from the car, and there is more than 1/8 inch of play in the input shaft. It is really apparent compared to the input shaft on the 5 speed, which permits no visible lateral motion. You can feel some play, but neither of us could see any movement.

At least I hope that is the problem. I am still going to have the drive shaft and half shafts reconditioned. I have no information on how old those universal joints are. They may appear to be fine now, but while I have them off I may as well make sure.

In any event, the 4 speed has bad synchronizers in second gear, and I hate the huge gap between 2nd and 3rd, so I wanted to replace the transmission anyway.

[Walter Moore]

Just so that I don't lead anyone astray with this thread, it turned out that I had several issues, some of which apparently masked other problems.

1. The old 4 speed transmission was bad, which caused part of the vibration.

2. Some of the old Universal joints were stuck, causing the rest of the vibration.

3. One of my brake calipers had a stuck piston, causing excess drag.

4. But the primary issue turned out to be a bad mechanical fuel pump.

Actually it might be the pump eccentric on the cam gear, but when I installed an electric pump back by the tank, the problem went completely away. I just removed the mechanical pump to eliminate the risk of rupturing the diaphragm and filling the crank case with gas.

The car actually runs better at all speeds now than it ever did. It used to be sort of rough and surge occasionally at idle, but now it is smooth all the way from idle to the red-line.