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.