kroznix23

Ok i got a 99 tacoma with the 3.4 6 cyl engine. 130,000 miles on motor. No check engine light. When motor idles down from high idle on cold crank up it acts like it has a cam in it. it is rough shaky feeling. What i have checked and replaced so far. Mass air flow been replaced, fuel pump has been replaced, throttle body cleaned, spark plug wires replaced. all plugs checked for carbon track and tips checked and seem to be ok. Engine isnt throwing a cel and all that has been done since it started acting like this. If i take and block off the return fuel line temporaryly boosting fuel pressure to like 85 psi it clears the problem up. The dealership said it was maf and was shocked and said they dont know what it is when that didnt clear it up. any suggestions? have also looked all scoped all around intake to listen for vacuum leaks and sprayed wd-40 to look for leaks and no leaks anywhere that i can find. U can also take and put hand over air box intake and if u do it just right to slightly lower air intake it cleans it up.

mt_goat

Welcome to YT.

My first guess is dirty fuel injectors, but I'm really just guessing. Might try a can of BG-44K in the gas tank.

BTW my 2nd guess is a dirty fuel filter.

hross14

Sweep the TPS switch--use a sensitive graphing volt meter/scanner if you have one. but thats what it sounds like. They have been known to go bad just enough not to throw a CEL yet make the truck act funny--they are only like 40$

Personally, once my vehicles reach that ~100k and change mark and things start neededing replacement--i do them all--O2's, MAF, TPS.....etc..

You just waste less time and get it done more soon.

also, just as a precaution, when i have plugs out and i am doing the major review of parts--do a leak down/compression test. Just to see where you are/health of the engine.

kroznix23

sweep the tps switch?? Where is that located and what should the readings be to indicate a good or bad one??

kroznix23

also wouldnt a tps sensor make it idle at different rpms or irratically? The rpms dont seem to change on the motor. problem gets worse as motor warms up and comes down of high idle. also u can take it down road and drive it and its like it dont have a problem at all. its only at idle u can tell. at first if u put it to the floor it wouldnt turn over 3500 rpms but then fuel pump was replaced and now it will turn as many as u want it just still has the idle problem.

kroznix23

ive been thinking maybe and injector or something but my mechanic doesnt seem to think it is that by how its acting. another mechanic says its a fuel pressure regulator but that was after his maf sensor guess wasnt right. as for the fuel filter it is replaced twice a year atleast even though ive been told toyota's fuel filters hardly ever need to be replaced. i have took it off and blown thru it and i can blow thru it just fine. doesnt seem to be clogged up.

hross14

There is no easy way around it--you have to start checking the system piece by piece. STOP GUESSING!!

The TPS sensor is on the throttle body on the side. I dont recall the values off the top of my head but do a GOOGLE for TPS Values Toyota or something like that--there should be articles to tell you the whys and hows.

Here is what i would do--

sweep sensors--all of them--URD Sells a nice box that will allow you to hook up your computer to your truck and read sensors. When you test them, take your time--for example--open and or move TPS/sensor test very slowly

Test Fuel pressure--do a search on "gauges" and get a good one

Test pressure regulator

What i trying to do here is to get you out of that frame of mind of guessing and or hunting--when you know something is not a problem then you can mark it off the list.

Also--at 130k or so your electric fuel pump starts to grow weak--the pressure regulator compensates for that and adjusts---until one day it dies. that may be why the truck goes above 3500 RPMS now

FYI--When a maf goes bad--the engine wont rev or 2500 rpms

There is alot i could cover--but it would be a huge post--Sweep the TPS and check your fuel system--then move on to harder items--getting injectors cleaned etc.....

kroznix23

Not actually guessing. Replaced plug wires cause i had a bad wire and was causing random missfire code. since have not had a code. Checked plugs after that cause motor wasnt idling right still. kinda feels like a missfire by way it shakes truck. then after checking for vacuum leaks with out any luck at finding one we pinched off return line to boost fuel pressure to see if it maybe fuel pump goind out and dropping pressure. Doing that cleared up the idle problem. took it for test drive and with pedal to floor wouldnt rev over 3500 no matter what. Replaced fuel pump. still acted idle the same way. but would then rev over 3500. Thats when dealership and another mechanic said it was maf and very common. replace them all the time at dealership for that. That didnt change nothing. After that didnt fix anything and blocking fuel return line boosts pressure we are thinking fuel pressure regulator. Truck has been hooked up to some hight dollar dealership diagnostic tools and they all say nothing is wrong. so not exactly guessing but if regulator doesnt fix then dunno. dealership said regulator is starting to go bad cause it is leaking back into intake. so we are definatley going to replace. i know fuel pressure when return line was blocked off was 85 psi but not sure what it was when it wasnt blocked but guess it was good number cause no one has said it didnt have enough pressure. all say it has good pressure. when we replace regulator and no fix then will probably take fuel rails off and check real good for trash. thanks for the replies and will definately look into the tps sensor more.

atistang

i don't get why they replaced an expensive component such as the MAF on a guess, yet didn't replace an inexpensive think like spark plugs

I swear when someone gets a lawnmower running they quallify for most shops

Check your fuel pressure. if it is fine just go ahead and replace your spark plugs.

if that doesn't work then check your tps voltage at closed throttle. most obd1 cars at closed throttle should be between .065-.085 or so, but try to find exact specs for your truck. then do a sweep and look for any spikes or drops in the voltage.

i'm going to guess its your FPR though, it is probably sucking even more gas through the vacuum line and that is why when you pinch it off it clears up a bit

mt_goat

I think they mainly do it for profit. It's easy to replace, probably pays good, and the tech can then spray a little brake cleaner on it and sell the old one on e-bay for $35. I kid you not, for years there was some guy near San Diego that always had used MAFs up for auction. I bought one from him and it came in a factory box (opened). It works perfect. My guess is this guy is a Toyota tech at a dealership in San Diego and always recommends replacing the MAF.

seaofnames

I've never had a problem with my fuel pump 'getting weak' and I have over 255,000miles on my runner.

Replacing all that extra stuff seems really excessive instead of spending TIME diagnosing it. Like others have said, check resistance on TPS and fuel system. Perhaps dirty fuel filter(that sounds like the best one so far). Dirty injectors are next.

Good luck!

hross14

I bet you $1000 cold hard cash that the resistance in your fuel pump is greater than when you first got the truck. Want to place the bet--it happens but we dont notice because the FPR "regulates" the pressure.

None the less--at 100k things will start to wear--maybe not anytime soon--but maybe when you are out in the middle of no where it will fail.

If i were you though and planned on keeping the truck i would get some kind of scanner that can read OBDII and not OBDI....take your time and do your test and you will find the problem--

kroznix23

i do agree with u about dealership techs. lol. Plugs were replaced with the recomended ngk plugs about 10 thousand miles ago and thats why they were just took out and looked at and not replaced. they looked great still. but are inexpensive so why not replace them again. Still got to check the tps. I know the fpr is sucking gas thru vacuum line cause u can see it wet in there so it is starting to go bad and will replace.

kroznix23

o and we called the dealership for advice seeing if they knew what it possibly could be since they do work on toyotas all day. The dealership isnt doing the work nor am i buying parts from them. I guess i dont plan on selling truck anytime soon probably why replacing stuff on it that may and may not have been needed really doesnt bother me too much. takes a lil money away from other stuff i might want but dont plan on getting rid of so its kinda just a lil pre maintenence on items that prossibly might be going out. lol. not that i just want to go replacing everything on it though. I appreciate all ur info though. Havnt had time to find out if its helped me or not yet but i will let u know when i do

koda4

Here's another suggestion. You can check is the IAC (idle air control) valve.
Do a search on how to clean it.

KODA4.

atistang

so lets see here, your fpr has gas in the vacuum line going to it, and if you pinch that line off then it seems to fix the problem

should we ask Mr. Obvious on this one?


replace your damn fpr, its like your truck is yelling out to you what the problem is

kroznix23

no not the vacuum line coming off regulator that we are pinching off. it is the fuel return line that is being pinched off.

atistang

either way if your fpr is bad, replace it. if your throwing parts at it at least start with the ones that you know are bad

kroznix23

yeah fuel pressure regulator is already been ordered for it. should get to do some more work and get that regulator on monday. ill let u guys know how it turns out.

atistang

after that i would pull your plus and clean them with a wire brush and brake clean, that is if they are black with unburnt fuel