496HO - Odd tach behavior
 

Guys,

Recently, I’ve noticed my tack acting strange. It will sometimes read high. The 496 is supposed to idle around 650 or so, and it will sometimes show as much as double that. It will usually tach out at ~5100 at full throttle and full trim, but, when this behavior is on, I can easily tach it to that at a little over half throttle (where cruising speed would be). It is intermittent, but when it’s on, the tach will read high across the RPM range. For example: Typically when I am planing, I will throttle it up to ~3K, and decrease throttle as it planes off and eases up on prop load, but the other day, with typical throttle lever positioning, it “revved” to almost 5K during this process (engine sound did not reflect this). I’ve also seen the tach jump from 650-700 to 1100, without any change to throttle lever position or engine sound.

Then, yesterday, while out, initially it seemed to reading correctly, but then started acting weird again. The tach reading and GPS aren’t matching up as I remember them, and I could once again tach it to 5K+ at considerably less than full throttle and no trim above cruising attitude. I also noticed that, by the end of the day, we’d gon through considerably more fuel than we really should have. Probably 50% more than I would have expected. Additionally, the heavier-than-usual transom soot was back at the end of the day. I had also noticed I couldn’t really get any more RPM than ~5100, even if I stuck the stick on the firewall, and we were about 6MPH short of what we typically see at WOT and full trim.

At first, I figured it was just a matter of a new gage, but then the fuel issue came to light. My concern is this: As I understand it, all the gages are connected to the ECM, either as feedback to the ECM, for warning purposes, or just the gage reading data being reported to the gages from the ECM. With the fuel map in mind, if the ECM thinks the engine is running at, say, 4000 RPM, when it is actually running 3400 (a typical crushing speed), is the ECM dictating more fuel than the engine really really needs? Off I just stab it, and the tach hits the “rev limiter”, even though the engine isn’t actually running that fast, does the ECM actually react and start “limiting” the RPMs?

Can I keep the TechMate connected while running and cruising, to see what it tells me?

Thanks. Brad.

I run my techmate while cruising around on occasion or when testing out a new build.

Sounds like the tach is going bad OR there is a switch on the back of the tach (under a cover) that could be shorting out. The switch changes the tach for 4,6, and 8 cylinder.

That sudden fuel burn would concern me. Check you fuel pressure at the rail while you are doing the tach diags. You might have Cool Fuel module problems.

Tach lightly gets it it's signal through SmartCraft and the Crankshaft Position Sensor. They can start to fail/go-bad before showing a code. It's not a bad idea to replace it.

I think he's running a Procharger so yeah, something may be up with fuel pressure. I'm sure it is setup with a different pump/regulator?

rVerify all grounds and connections etc. If all that checks out replace it before it totally malfunctions and perhaps damages something else in the process. I would suspect the ECU would not like a component connected to it with a short or some other weird electrical problem.

Techman,

Fuel pressure seems fine. At least if the gage on the dash is to be believed.

Thanks. Brad.

Griff,

Although I was unaware of the switch you mention, this was my initial thought. But then I begin to wonder if it might be getting bad info from the ECM or some other component. Any way to confirm on way or the other?

I suppose the cheapest route is to verify connections and/or replace the gauge first, right?

Thanks. Brad.

Knot,

Yup. ProCharger w/ HP fuel pump and boost reference regulate. All seem to be working properly, according to the boost and FP gauges. This is why I am asking about the possibility that faulty tach feedback might be influencing the fuel map from the ECM.

Thanks. Brad.

Brad, the tach in my Laser(502Mag/mpi) got erratic as you have described, but it did not affect in performance in any way. I sent the tach in toFaria for a rebuild so I could keep original gauge with Powerquest logo. The idle RPM always shows high I think, but this year it started skipping around in the 3000-3500 range while cruising. I pulled the dash and rotated the cylinder select switch back and forth through the range and it seemed to fix that problem. (must wipe/clean contacts in there)
The switch is on back of tach, just make sure to get back to original position. If I have more problems I’ll replace the tach. Not real happy with the rebuild.

Guys,

I validated my engine's RPM with the TechMate over the weekend. It seems to be reading about 200RPM low at about 3500RPM, and maybe 300RPM at 4500. Is there any calibration that can be done? Maybe a gain metering for needle position or whatever?

I have not done the selector switch "wiping" yet. I plan to, just haven't taken the time to remove the gage panel to get to the selector switch yet. I did, however, do the old "gas gage knuckle tap" test and, wadayano... It worked! Over the weekend, we were out cruising, and it was reading significantly low, so I tapped on the gage face with my knuckle, and BANG, the needle jumped up about 7-800RPM, with no change in physical throttle position or audible engine speed change. What do we read into the results of this very scientific experiment?

Thanks. Brad.

It could be as simple as a loose wire connection on the back of the tach.

Griff,

I'll definitely be inspecting connections while I'm behind the gage panel.

Thanks. Brad.

What tachs do you have?

My Livorsi tach's read about 200-300rpm low on my 496HO's.

Emailed Livorsi and they said they can rebuild/recalibrate them for $88/Ea.

Concept,

Gaffrig.

Thanks. Brad.