Weird tach behavior after Sea Foam

[fleck]

So ever since I Sea Foamed the truck she seems to be running a whole lot better and like I said before basically ripping through the higher RPM range like it's none of your business. But I have noticed something strange happening that I hadn't quite noticed before.

First, when idling, the tach 'bounces' or 'spikes' up just a tiny bit real quick, sometimes it'll stick a little and sometimes it'll drop right back immediately. It doesn't feel like this is what the engine itself is doing, but with a spike of only like 50 or 100 RPMs it makes sense that I wouldn't notice. I was wondering if this could mean that my IAC valve has always been clogged up and is now showing signs of being bad.

Second, it seems to hop between RPMs in the high range, like it will seem to jump from 3.8K to 4.2K then hop right to 4.8K and it will stick at 5K for a while before it opens up beyond 5. The only part of this that is noticeable while driving is the stick at 5K, it takes a couple of seconds to rip up to open throttle once it's there. I have no idea if this could be a bad TPS issue, hopefully not a bad cam sensor unless those just keep the truck from running.

Any help/ideas would be appreciated.

 

[jmg8550]

It might be the instrument cluster itself. If you want to check for rpm fluctuations on the engine, try a timing light with a tach. Or maybe temporarily hook up a aftermarket tach to it. does the gauge seem to only do it under heavy acceleration load?

 

[fleck]

It might be the instrument cluster itself. If you want to check for rpm fluctuations on the engine, try a timing light with a tach. Or maybe temporarily hook up a aftermarket tach to it. does the gauge seem to only do it under heavy acceleration load?

OK cool. Tomorrow I'm gonna be visiting a mechanic I know who worked on my truck a while back, I'll ask him to do this for me (in addition to the other couple of things I need), so we can see if the engine is actually behaving this way or not. Sounds like a good place to start.

 

[Viper Command]

when you did the sae foam did you pull the idle air control valve and clean it?
this could be your problem.

 

[rurouni20xx]

my trk when cold the speedometer needle bounces to 85 then shoots back a few times, i fig it was a loose connection on the instrument cluster or vehicle speed sensor, or corroded wire. this may be the same thing w/ your tachometer. the iac will make the idle actually jump up and stay there or even worse hold the throttle almost wide open or have cruise control, i changed mine when it did it, but to get it to autozoo i had to disconnect it and idle w/ my foot at lights since the engine wanted to surge uncontrollably. if the iac was the culprit you would know about it to say the least. your second problem sounds like when my computer cant tell if im full throttle or not and wants to act stupid when changing gears. it will be in 1st to 4 k then go to 2nd and drop to 3/35k then reshift to first and run out to 48-5500 and finally go into second at 47 mph when its supposed to then run out normally to 55k at 75mph to 3rd and then go to the limiter (beyond 85 to between D/OD and bounce limit out) til i let off the gas and go into 4th and cruise at the limiter but when you hammer on it to accel it goes into 3rd and bounce limits and will not go any faster. found that out on a hard run to blow out the motor and since then i havent gone fast enuff to bounce the limiter again, but get close on occasion . the only thing i can think of on the tranny "reshifting" is either a bad tps reading or stretched throttle linkage making the throttle only open to 80-90% and the computer doesnt read that as WOT to keep into first. if its a manual transmission doing this i dont know what to tell you.

 

[fleck]

Viper do you mean it could be a problem if I DID clean it? I did not clean it, I just ran the Sea Foam into the PCV valve, crank case and fuel system (the full treatment).

rurouni, thanks, I'm not having a problem with the transmission itself, I mean I guess it could be since it doesn't hop around like that when I rev it up in neutral, but yeah it is a stick shift and it's not doing anything weird itself, I just see the RPM gauge hop from place to place (upward only) while getting up in speed in gear and I hadn't seen that before I did the Sea Foam. I just noticed that it does it through the whole RPM range too not just the end.

 

[Viper Command]

well the iac is not your problem.
when i do an injection service i clean the iac every time to prevent them from gitting cloged.
look around the pcv to see if there is a loose wire, but it could just be a coincidence in timing.

 

[fleck]

I've been observing the tach more and more and I think it's just dying, it's bouncing up a little from time to time while driving at a steady speed. Maybe it's one of those wacky coincidences.

 

[rurouni20xx]

wiring issue like mine, loose wire or short to ground somewhere (bare wire) causing it to peg out, check your wiring where it goes thru the firewall and make sure the wires arent chafing. also trace your tach output wire back to the gauge, if the wires are fine then its something in the gauge or the output.

 

[shizzy]

did you replace the plugs after you sea foamed it? I had an old car do the same thing. fresh set of plugs cured it.

 

[rurouni20xx]

mine does it all the time and have changed the plugs several times, now if the seafoam has affected the resistors in the plugs that is new to me. the reason you use resistor plugs is to get rid of noise in the electrical system that you can hear when the radio is on and the motor revs making a high pitched whine in the speakers. what it also does is clear up that noise in the rest of your elec system, making tachs jump, speedo jump and the rest of the gages from going nutz. that could be the case and if it is thats interesting, but generally when most ppl do tuneups and possibly seafoaming they change the plugs and oil and everything else as well, or so i should hope.

 

[fleck]

Well I changed the oil a week before doing the Sea Foam. A lot of people use Sea Foam constantly, I've never been instructed or informed about changing plugs, but they were changed at the same time the oil was changed... they were years and years old, black and had drooped to a gap of about 075 said my mechanic, wondering how the truck was even still running.

This truck was slightly abused and ignored the handful of years before I got it, the previous owner tried to pimp it out but it got stolen. After a while of driving it I found that there were huge cracks on the underside of the intake right before the manifold, we took off the intake and the inside of the manifold was dark brown, we've done several intake cleaning treatments, and she still makes popping sounds when I rev her up (no it's not engine knocking, she's literally still shitting out little by little all the crap that got in the intake system over possibly years of being driven with holes in the underside of the intake).

 

[fixizin]

Fleck, I too vote instrument (or wiring connecting to dash) issue; i.e. with a manual tranny (and assuming your clutch is not massively slipping) there's no way your engine could ACTUALLY be skipping around the RPM range (as you described) without being readily "sensed" by your "butt dyno", lol. You'd have a diagonal stripe across your chest from G-forcing off your seatbelt all day. ;')

Oh yeah, what mtn said: do NOT run that batch of crankcase oil too long after SeaFoam, or ANY fuel injector cleaner, etc... it breaks down oil. i.e. figure you run out the tank of gas you poured it in, and then ONE more tank AT MOST, then change... read the label... there's a warning to that effect on there... in REAL TINY print...

 

[rurouni20xx]

i wouldnt go 300 miles on it (a tank of gas) id do like radiator flush, add it plus water, ride around 20 miles, drain, refill w/ fresh predil on radiator and fresh engine oil, do a fuel filter and serpentine belt, clean and oil air filter, plugs/wires and cap/rotor while i was at it. not to mention check tire psi and all lights/switches. preventative maintenance is the best maintenance!

 

[fleck]

Did you read my sig? All of that stuff minus the wires and cap/rotor is brand spanking new. I'll take the advice on the oil change and I'll check the spark plugs to make sure they didn't get messed up. It only smoked for about a minute so it was actually not that dirty, I'd done a ton of intake cleaner treatments before and plenty of Lucas has flown through the gas tank. I don't think the Sea Foam has obliterated anything, I sure hope not!