Since installing the new engine in my 87 the tach scatters after about 3,000 rpm. I've heard people talk about a tach filter being a possible cause. Where is this tach filter and do you think it might be the problem?

 

You too eh?

I don't know if you like a STOCK interior... I am installing an auto meter mini-tach with a shift lite very soon. I don't trust the stock tach. That might be worth looking in to. If you didn't see it yet, I got second place in my first show today! Wait'll it's wet sanded :D

 

The tach filter is bolted to the rear of the drivers side head. Has a single wire coming and going from a capacitor looking thing. If this is bad or going out, it could also be a cause of your pinging. The computer could be advancing the timing by what it's being told the rpms are, which are rising at a much faster rate then they really are. I had this happen on my '85. It ran crappy until I got the filter corrected. If it's not grounded good, it will also cause this problem. They are no longer made by GM and some people take advantage of it and sell them on ebay for 20-30 bucks for a dollars worth of parts.... If you know anyone electronicly inclined, you can build a new one really cheap. Here's the schematic from GM on the tach filter for '85-'91 model cars:


http://www.corvetteforum.net/c4/marc...achfilterj.jpg


on my car, I forgot to mount it on the head when I put the new motor in, and that's why it didn't function correctly. Once I mounted it and it was properly grounded, it worked fine. I did have alot of detonation and pinging while it was loose. Something to check out....!!!

 

JD, OUTSTANDING! What kind of show did you enter? Show -n- Shine or Show -n- Go? I'm very tempted to put a 3" Auto Meter with a shift light right in front of the stock tach. I, like you, don't trust the stock tach, besides, they always seem to run behind/slow. They're fine for routine driving around, but don't make good race tachs. The big 5" tachs are just to much though.

Craig, hummm...I wonder if that could be part of my ping problem! I don't really get the connection between the tach filter being grounded and the ping, since my car is pinging at almost all throttle input levels, even well below the point where the tach goes away, but you may be on to something! I'm pretty sure that my tach filter is hanging loose behind the engine. Getting to it to try and bolt it back up may be a real challenge, but I'm going to give it a go!

Thanks for the suggestions...I gotta get this darn ping cured. Right now I don't think I'll try the dyno this Thursday because of the ping. Even though I've done several 0 - 150+ full throttle runs with it the way it is! :eek: What happens when an engine detinates? Does the fact that it hasn't damaged the engine yet mean that the detination is not bad enough to casue a problem, or am I flirting with the ?

 

Here you go EZ.... Yup, I think it was show and shine. I entered Personalized Stock.. Basically stock car with my own modifications(READ LOTS). Go for the mini-tach. Mine goes in Tuesday. Stainless Steel Brake Lines tomorrow :D Here are a few pics for you.

 

Jeff,

If you can locate the tach filter, just wrap a wire around the mount and run it to the negative battery terminal to ground it. I did that until I could get in there and mount a new one. worked great!

I would be really nervous running WOT with alot of pinging. It's just a matter of time before something breaks.

 

The tach filter is not makeing it ping.A ping is the valves slamming against the valve seat in the head.You probably will start loosing compression from the ongoing effect.Have you changed the knock sensors & the ESC.If so then you may have a bad distributor.

 

True, the valves slamming against the valve seat in the head is the actual pinging noise, but what's causing the valve to slam into the seat???? Too much timing advance is one thing that can do that. What is something that can cause too much timing advance? If the tach filter is messed up, and the RPMs being reported to the ECM are higher than what the engine is actually doing, then the ECM will use the wrong part of the timing table in the chip. Timing in the table increases as RPMs climb. Say for example the table looks like this:

RPM is on the left, across the top is load.... col1 representing light load and col4 representing heavy load. THESE ARE JUST VALUES I MADE UP TO DEMONSTRATE!!! These are not actual numbers being used on any application that I can think of!!!! Please just bear with me and read below!

RPM: - col1 - col2 - col3 - col4

1000 - 26d - 26d - 26d - 26d
2000 - 30d - 30d - 30d - 30d
2400 - 36d - 34d - 32d - 32d
2800 - 38d - 36d - 34d - 34d
3200 - 40d - 38d - 36d - 36d
3600 - 44d - 42d - 38d - 38d
3800 - 46d - 44d - 40d - 40d
4400 - 46d - 44d - 40d - 40d


So, in this example, lets say the motor is running at 2000 rpm but the tach filter is messed up and the RPM's are being reported to the ECM as 3500. The ECM is now adding anywhere from 6 to 10 degrees more timing than it should for that given rpm/load. Hmmmm.... would that cause pinging? Maybe. What if the driver stabs the throttle? Well, in light load situations, the timing is advanced for fuel economy. At WOT the total timing is alot less than light throttle at the same rpm. Well, because the tach filter is messed up and we're already 6 to 10 degrees advanced, add to that the fact that we're at part throttle, which is another 6 degrees more than WOT timing.... and for a split second the timing is 12 to 16 degrees higher than it should be..... that's not good. Are you getting what I'm trying to explain here? The tach filter, in my experience, can certainly mess with the ECM's ability to properly time the motor. And we won't even go into how some ECM's have a highway spark mode that is enabled to advance timing even further under certain light load conditions.... Most early Corvettes don't have this feature enabled.


Now, if the tach filter has no correlation to the timing being reported to the ECM, then all this is blown out of the water!!! :D There is a slight chance that the filter is on there only for the purpose of the guage, but I'm not sure. There is only a single wire coming from the distributor through that tach filter to the wiring harness for RPM measurement, so I'm guessing the ECM and the Dash both use that wire. I don't have my Helms manual to verify....

But, in my '85, when the tach filter was messed up, I did experience more detonation and bad running than after I fixed it.

 

Hi,
Can anybody confirm if the ecm does receive the information through the filter. In my 90 the distributor has two wires coming out next to each other, one goes through the filter and the other goes straight into the harness.
thanks
Steve

 

looks like you've invested a lot of time, know-how and effort - you've probably got fairly recent plug wires on there (?)
i've seen multiple electrical gremlins disappear with the replacement of failed spark plug wires (especially the wonky tach).

 

Pic of 92 with rare dual tach option...with room for a harlen shift light.

 

Craig,

Thanks for the indepth info. I've been in Seattle for the last couple days and haven't had time to dig into the filter issue yet, but rest assured that I will very soon. If that has no effect I'm going to try a set of cooler plugs as recommended by a few knowledgable people.

JD,
Congrats on the show-N-Shine! Looks like you had lots of competition. My personnal opinion...don't do the black wheels! In humble opinion you'll kill the looks of that sweet looking ride! But that's just my opinion :p:

 

thanks EZ........

See my current NIGHTMARE with this car! We're not alone. Mine runs like S*(* too now!