HELP Please, Engine rpm issue

First time actually posting on this site. Hope this is the right place.

I got a 2005 Tacoma Prerunner SR5 4.0L V6 with a lot of miles, ~273k.The issue started out as a kind of random sudden drop in idle rpms. Would sit at around 700 and drop to about 500 randomly and go back up to normal. No check engine light has ever come on. That was roughly a month ago. Now, the truck “sputters” when I accelerate - but not when I got it all the way to the floor; still having the idle issue.

My brother and I called up a Toyota master mechanic and we all ended up at it’s gotta be fuel system related/mechanical. Put a can of sea foam in and drove it like we stole it hoping to clear out clogged injectors if that was the issue. Hooked the scanner up to the truck to check the response of the pump and messed with flow rates but there was no response. Then we swapped the fuel pump for a new one - I understand the fuel filter is part of the fuel pump so a 2-fer-1 special. Spark plugs were replaced too, back around the start of the issue, also because they had ~30k miles already. Checked the old one’s for oil or any “burns” and nothing. Cleaned and wiped down the throttle body. MAF looked spotless so I wasn’t too concerned- maybe I was wrong. I cleaned the air filter - but didn’t replace it. Did a once over of the whole engine bay and didn’t see any cracks/hear any hissing from any of the lines. Money is getting kinda tight. Was going to check the coils next with a multimeter and see the resistance values. At this point might just end up swapping parts from my brothers truck and see if that fixes anything, figure it out by dumb luck maybe…

Looking for any suggestions or ideas. Also feel free to ask about anything that I may have skipped over.

thank you in advance!

 

What kind of spark plugs did you use though?

 

What do fuel trims look like idling?

Even though the engine light isn't on have you checked for codes?

 

2015WhiteOR - did all 6 with Denso Spark Plug Number K20HR-U11 from O’reilly’s. But to be clear, I replaced them after a week or so of rough idling issues.

Dm93 - not sure what the fuel trims were when he looked the first time, but I’m heading over to my brothers house this weekend so we’ll check it. And when we checked for codes a few weeks ago there was nothing. Think we’ll check that again too. I got a cheap reader that connects to an app but nothing showed up a couple days ago.

 

Maybe the MAF sensor is bad? try switching out the one from your brothers truck and see if that changes the behavior.

 

Clean the MAF...

 

Doesn’t sputter when pedal to the floor?

There is a fuel pump resistor in these trucks. Low load and low rpm it goes through the resistor.

High load and high rpm it bypasses the resistor.

Plus, if expect the engine to die completely if it were the resistor.

But we really don’t have enough info to help much.

The best data would be graphed on a scan tool as the problem happens.

 

Or just bypass the resistor as a test.

Unplug the fuel pump resistor and place a jumper across the connector terminal.

If the issue goes away the issue is either the Resistor or the Fuel Pump relay.

 

Gonna clean the MAF tomorrow evening, and if need be swap it this weekend. Also gonna plug my little reader in and record it on my phone as I drive around tomorrow. Hopefully get some better info.

Definitely gonna check out that resistor too and see if that does anything.

will post an update as soon as I try all that.

 

So, haven’t gotten a chance to actually get into it, but was able to drive around with the obd2 reader. Long term Fuel trims for 1 and 2 are roughly 0 to 4%, Maf reads 3.7-6.8 at idle and hops on up to 15-25 around like 1.5/2k rpms and all the way to 60 when I step on it.

got some crazy weather going on and no garage… but when it lets up I’ll clean the MAF and test the resistor.