Speed limiter or tachometer problem?

#1

At VIR this weekend I had a scary moment on the back straight when I thought my motor let go. It made sense in my mind because I had just done some head work.

I was moving along a little quicker than I had in the past and all of a sudden the car sputtered several times (right before the braking zone). After the brake zone, I took it easy and everything felt fine. Came around again and the same thing happened.

I realized after that, the tach was not working. So my question is: Did this happen because the tach was not working or is there a speed limiter on these cars? Do I have the wrong ECU?

Question 2 is how do I track down the problem with the tach?

Great time and great weather at VIR by the way!

Thanks,
Jason

#2

My 87 325is used to hit a limiter at 124mph/200kph which is about 5200 rpm in 5th.

#3

The tach is just an instrument fed a speed signal from the DME, It’s operation, or even whether it is present has nothing to do with the sputter.

Very late production E30 DME’s had a speed limiter. I think the Bosch number of those is a 525 or maybe higher.

If the sputter only happens at high rpm and WOT I’d first suspect the fuel system.

#4

It actually only happens at the top of 5th gear. I don’t have a data logger so I’m not sure how fast I’m going (can’t see the speedo because of the wheel) and now the tach isn’t working so don’t have any indication of speed. But on the back straight at VIR I’m moving along as fast as I can.

It has to be a speed limiter if the tach is not involved because I ran it up to the rev limiter in other gears just to make sure that I wouldn’t get that same issue. Thanks for the DME number, I’ll check that. I do remember the DME not being the one that everyone says to get so that may be the issue.

As far as the tach, there is a bundle of wires under the dash and some are cut, some are spliced. Most of them look to be stereo wires but I’m wondering if one is the tach. Anyone know what color that wire is? There is a white wire in the midst of all of the odd color stereo wires that I’m wondering about.

Thanks for any info.

Jason

#5

Because it’s only happening in 5th gear does not rule out fuel issues. Something in your fuel system could inhibit fuel flow/pressure at high speed in 5th gear, but work fine in lower gears. The time spent under WOT throttle is longer in higher gears and the fuel system may not be able to keep up above 5200 rpm but may be able to keep up in lower gears. If you are running out of fuel, your engine is leaning out and likely detonating. It doesn’t take long for detonation to destroy piston rings and other things in the combustion chamber. On my first engine, I had a rusty fuel tank that caused the fuel pump screen to clog up with rust particles and restrict fuel flow, cause detonation, and destroyed my piston rings in just a couple runs. My symptoms were very similar to yours… I recommend ruling out fuel issues before you move on to other things. Check the condition of your tank for rust, but keep in mind that rust will settle to the bottom and may not be visible until you shake the car up. It took about one lap for my fuel pump screen to clog up an restrict flow and only a few minutes for the rust to settle in the tank. Also check your fuel delivery by using the method described in the Bentley; pull the bottom fuel line from the fuel rail and pump gas into a container to determine your rate of fuel flow.

#6

A while back I brought a bunch of ECUs to the dyno, and one of the non-favored ones (524 or 525 if I recall) and found it would rev to 6500 in the lower gears but for fun the dyno operator shifted to 5th to see what happened and it initiated a soft cutout (felt more like ignition breaking down than the hard cutout we see at the rev limit) at about 120-125 mph (didn’t keep the results and don’t recall the exact speed), but there was definitely a speed-limiting aspect to it.

Interestingly, the power/torque curve was pretty spot-on with a 173 right up until then.

#7

I had a similar problem at VIR last summer. To rule out fuel as a possible cause I found some extra fuel line and fastened a FP gauge to my windshield wipers so I could see it. Than I did some laps and the next time the symptom occured I could see that FP was fine.

There’s a pic of that here the VIR thread from last summer.

The problem went away after I gapped my spark plugs (I’d not been the one to install them) and torqued them in tighter. My perception at the time was that the plugs had not been in tight enough.

Now I have a FP gauge on my dash. No more fastening a FP gauge to the windshield wiper for troubleshooting sputters.

What you’re seeing is not a tach problem. Like Jim said, the tach is a symptom, not a cause. My guess is that it’s not a speed limiter problem either. SpecE30’s do what on VIR’s the back stretch, about 125? That’s just not that fast.

#8

[quote=“Ranger” post=54703]I had a similar problem at VIR last summer. To rule out fuel as a possible cause I found some extra fuel line and fastened a FP gauge to my windshield wipers so I could see it. Than I did some laps and the next time the symptom occured I could see that FP was fine.

There’s a pic of that here the VIR thread from last summer.

The problem went away after I gapped my spark plugs (I’d not been the one to install them) and torqued them in tighter. My perception at the time was that the plugs had not been in tight enough.

Now I have a FP gauge on my dash. No more fastening a FP gauge to the windshield wiper for troubleshooting sputters.

What you’re seeing is not a tach problem. Like Jim said, the tach is a symptom, not a cause. My guess is that it’s not a speed limiter problem either. SpecE30’s do what on VIR’s the back stretch, about 125? That’s just not that fast.[/quote]

traqmate says i hit about 121 before braking for 14, but then i don’t have a nasa-ma motor :slight_smile: