Acceleration fluctuation issue

Greetings Taco peeps. I'm looking for info on a problem my truck is having. Under acceleration, usually noticeable above 20-25 mph my rpm fluctuates up and down as if the throttle is being cut off momentarily every second like clockwork. I can speed up but the wavering nature is very annoying and happens to everyone who drives it so it’s not operator error. The dealer did a tune up replacing my air filter, spark plugs, and cleaning the throttle body. Now they claim I must have put bad gas in it. Since it’s been happening for months now, I only use good gas stations, and I’ve taken it from D.C. to South Carolina and back here to VA so that isn’t an issue. Now the dealer seems stumped. I mentioned it seems like an ECU issue, and that it didn’t start until I took it in to have my pedal cut from that unintended acceleration issue. Has anyone else had this problem? I searched a bit and didn’t find what I was looking for, I’m hoping someone knows about it.

 

Sounds to me like a vacuum leak or some kind of stoichiometry problem. MAF sensor plugged in? Try cleaning it maybe? Is your TB bolted down tight? Spark plugs have correct gapping? Any of the things they did to your truck can cause stumbling.

Are you an auto or manual? If auto, try leaving it in one gear and see if it has anything to do with what gear it's in or maybe if it's RPM related?

 

I'm having the dealer do all the work since that's why I got a warranty on this one. I'll ask about the MAF but the TB was just worked on so I'd assume it's bolted down correctly. Spark plugs were just replaced, same issue so that's not it. Auto and it happens in multiple gears so the tranny seems fine to me. Doesn't happen or isn't noticeable at low RPM, above 2k is when it becomes an issue.

 

Faulty coil pack perhaps? These can be a bugger if they go out because they don't always trigger a code. The 1st gen Honda Fit had bad coil packs as a somewhat common failure. The usual remedy was the dealer putting in a spare starting on #1 and swapping it with every existing pack one at a time until the hesitation disappeared.

Vacuum leak on a MAF system usually shows at the lowest manifold pressure, ie idle, so your describing the problem getting worse under acceleration and over 2k rpm probably throws that out.

 

TPS sensor....

 

Not sure how the tire pressure monitoring sensor will cause this, and somehow not throw a light

 

Try cleaning the MAF.

 

Did the dealership do actual troubleshooting and diagnosis, or did they just simply do a tune up and send you your way? Is it jerky or a subtle fluctuation?

Maybe your fuel pump filter is clogging, maybe dealership can service that under warranty. If not do a walboro 255 upgrade from URD if you can figure out if you have fuel delivery issues.

Maybe run a bottle of ISO Heat (red bottle) in your gas tank in case there is water in your system.

 

The OP's problem sounds way to specific to be something as broad as a fuel pump or fuel delivery problem.

 

I had the same problem but my rpm did not fluctuate, my engine would start to miss between 20-30mph. I was actually thinking about changing the fuel filter but the pump assembly cost $500 so I decided to run some Star Tron fuel treatment instead and change gas stations. The problem is now gone, so the bad gas theory might be true. I put the treatment in three consecutive fill ups and now I only buy gas from Shell.

 

Just because the dealer did the work doesn't always mean it was done correctly. There are plenty of stories here on lazy techs not doing the job properly. Just give everything they touched a good once-over. If you find something, you can either fix it yourself or take it back to the same flunkies that screwed it up in the first place.

 

Yup, that was the intent. Brain -> finger fart

 

I mentioned the coil pack to the dealer to check out as well as the MAF and vacuum leak ideas. I keep telling them to check the ECU since the problem started after going in for the unintended acceleration recall.

They might during the next round of testing. I need to go in and talk face to face to get them working on it faster.

Wouldn't a clogged fuel pump filter or something in the fuel lines cause the fluctuations to be more random? These fluctuations or dips in power happen like clockwork which is why I'm telling them to look at the electronics. Plus I use only good gas stations like Shell, Raceway, and our base gas station. Plus I've recently filled up out of state with the same issues. I won't upgrade it, I paid for a warranty so I don't have to deal with this kinda stuff.

Again, many different stations used over several months and various states.

I agree, and they seem to be trying to come up with simple solutions and give it back without fixing it. I'm gonna have to go in and show the mechanic what I'm talking about because they seem stumped. Once I show them hopefully things will start progressing.

Thanks for the replies fellas. Figures when I'm ready to try and sell it things start going bad (they're fixing a couple interior issues as well).

 

Does it do the same thing on cruise? Try it out on an uphill or use the cruise switches to accelerate if you haven't yet.

A suggestion that is a bit out there, but was one guys issue, and sounds similar. An intermittant or out of adjustment brake switch? If the ECM sees a brake applied signal it will cut the throttle.

 

check to make sure they replaced your plugs, air filter. Also if you can pull the spark plugs and check the gap. Usually the plugs are pre gapped but your issue is that your getting drops in acceleration. Check the vacuum lines for leaks, clean the maf. Anything problems at higher speeds?

 

Problem solved. Dealer had to pull the gas tank, drain it, clean it, and put fresh gas in. They said it smelled funny when draining it and looks like I had some bad stuff in there. Not sure what though. They said they smelled my tank and other gas tanks to see if there was a difference and mine had a strange odor to it. Just figured I'd post up in case anyone else has this problem.