Bouncy Tachometer

TBerk · August 9, 2018

My good friend, who I help keep running, has a 1993 C2500HD- the one w/ the TBI fuel injection on a 5,7L block & the Eight Bolt 2WD hubs/rims.

It's been running ok for a long time w/ more than 430K miles, mostly freeway, on the OD.

It does though have a few nagging issues, one of which is the tachometer bounces up and down erratically while the motor is running.

It's most pronounce when the RPMs are low, like around 1K-2.5K, but never really goes away.

The truck also has trouble sometimes pulling up-grade without getting some pinging from the motor. (Usually running 87 roz/ron octane...), I mention it in case the two things are related (Jumpy Tach & slight but persistent 'ping' going up steep grade on the freeway) in case they are related.

silveradosid · August 10, 2018

if the tach is getting a poor signal then so is the ecm check wiring for poor connections might have to replace cam sensor

no HD · August 10, 2018

TBerk, Congratulations to your friend still going fairly strong at 430K ! Thank you for helping him out.

You could hook up a dwell/tachometer and see if that meter is bouncy. If the test meter is not bouncy, then you know the pickup coil, module and ECM were OK.

As far as the pinging issue, with that many miles you may be looking at a loose timing chain or carbon built up in the cylinders. I'm thinking that during the pinging episodes there is probably some blue/white smoke out the exhaust. Assuming the EGR, ignition timing and the 87 octane fuel is OK.

Edited August 10, 2018 by no HD

TBerk · August 22, 2018

Thx for the replies; we just got in from a 200 mile run up the road.

On the flat freeway the truck ran pretty good; no smoke is ever seen from the Exhaust, even when it's not acting at it's best.

At one point we where on the 'snake trail' (twisty mountain road, single lane in each direction) and due to road reconstruction, had to wait a long time for the other trafic to pass by. Driver shut the truck off while we waiteddd & when we got back going again the motor bogged a lot and ran real rough. I'm supposing it was a heat soak issue (It got up around 100F today).

We kept pressing on and it settled down and ran OK for the next 30 mins until we got back to Base Camp.

OTOH; the end of the tail pipe shows a pretty decent build up of carbon. I'm not thinking it's running rich, I'm thinking it's because (at least) some of the cylinders aren't burning all the fuel sent to them, I'm thinking Spark at this point.

I'm going to pull the plugs this week and see how the ones back towards the firewall are doing...

(edit- I thought I had sent this reply, but I suppose I hadn't.)