MAF sensor questions for those who replaced theirs

It has a vaccum leak somewhere in it. Not enough to effect braking, but enough to cause a drop in vaccum pressure in the engine that effects idle and acceleration until the ECM can compensate for the difference.

 

I can watch my vacuum readings from the ECU while I drive. What should I look for to determine if this is the problem.

Could a vacuum leak in my EGR modulator cause the same thing? That is a known bad part that I'm hopefully replacing today.

 

well I just ordered the upstream o2 sensor and the MAF sensor new oem parts so that will be every sensor not internal to the engine has been changed.

I just saw a youtube video saying the engine coolant temp sensor can cause rough idle as the ECM tries to raise idle speed to help cool the engine if it thinks its running hot but if the sensor is doing "strange" signals it can cause the ECM to make wide fuel range changes to compensate that can cause rough idle.

never in my life have I seen such BS systems where it is so complicated and interdependent that nothing ever works the way it should, technology sucks

 

Both cases in this thread sound like vacuum leak problems. If you want to throw money around, throw it at vacuum hoses, they're cheap.

Check the intake boots (all the rubber couplers & main boot from airbox) very carefully. As in remove them, and inspect all the folds for any cracks.

Use a mister to spray water around all vacuum hoses or potential leak areas. If the idle drops momentarily, then you've found your problem.

Virtually all the sensors have testing procedures in the FSM that allow you to determine if they're operating properly. Most tests only require a simple voltmeter. In addition the check engine light would light if there's a serious problem with any part of the emissions system.

 

I agree with you, but I have hosed this thing down with carb cleaner and had no change in idle. So I suspect if I have a leak, it's somewhere internal like the brake booster or EGR modulator, or maybe in the EGR valve itself.

What size vacuum lines does this thing use? I can't find any information on the smaller ones other than maybe 3mm or maybe 2.5mm? I know they must be metric, which is not something I can get locally. I'd cut one open and measure it, but then I'd be without my truck while I order new ones.

I'd replace them just to rule them out if I knew what size to use.

 

Most auto parts stores will carry a size that's exact or very close. If you're able, take the truck to the auto store - remove one of the hoses and take it in. Keep in mind, the ends of the hoses stretch a little after use so they'll be a little bigger than original.

As for the intake boots, remove them & check them out. In particular the one that connects to the airbox/maf sensor. This rubber hose connects a stationary object (airbox) to a moving one (engine), so it's constantly flexing. After 15+ years it's likely to be on its last legs.

 

None of the parts stores around here carry metric vacuum hoses. The ends of mine are so stretched that they have to be cut in the middle to measure them anyway.

 

same here, I replaced anything that wasn't soft and looked new, first thing I did.

that's idle issues troubleshooting first thing to check

no leaks anywhere

mine changes "after the fact" so stop at a light and minor rough idle (what I would call just a normal I4 engine) then after about 15 seconds it gets rougher and starts acting like it wants to stall they all by itself it runs better again.

mine acts like the ECM doesn't know how to idle the engine correctly or its getting sensor readings it doesn't like so it is "ranging" through different fuel mixtures.

@ MrRiverMan - go by the dealer and you can buy the vacuum lines you need, they aren't any more expensive then the parts stores (well not much anyway) but they have the right sizes to replace any old hoses but if you did the carb cleaner everywhere then you don't have hose leaks

 

As you mentioned, the EGR could be open at idle. Or not fully closing if no P0401 is being thrown.

That said, if you haven't already, double check to ensure the vacuum hoses are plugged into the correct ports.

This thread has some nice pictures, not Tacoma but familiar engine.

 

I did find this little bit of info or replacing the ECM to fix it but its says nothing about my 99 model

http://www.askatech.com/AskATechLive/AatFileShare/References/FastTrack/TA141.pdf

 

ok I replaced the MAF sensor and the hesitation seams to be "mostly" gone.

there is still a "flat spot" in the 1500-1750 range but I think that's normal for these engines and I cant detect any stumble or unevenness to the throttle input like before, however we had a cool front move in so its a lot cooler right now which might change things when its hotter.

I didn't bother to reset the EFI because it seams pointless when it takes just as long to reset itself from defaults as it does to reset itself if just left on its own, so I will see how it runs in a few days. first impressions are that the hesitation was fixed by the new MAF sensor.

I still have the rough idle going on but not as bad and maybe that will sort itself out as well after the ECM resets to the new MAF readings.

I haven't put in the upstream o2 sensor yet, perhaps that will help with the rough idle but i'll wait until I can see how the truck settles in with the new MAF before making any other changes to it.

I like to know "what" did something good or bad and when you go changing a bunch of parts all at once you don't know what did what.

 

keaker, now that some time has passed what is the outcome of this? Did the maf end up fixing the problem? About a month or two ago my 2.7 started having a similar issue where its hesitating/losing power upon acceleration in the 1700-2000 rpm range. Also on mine I noticed the problem seems worse on a cool engine, it seems ok once its at operating temp. I've been trying to chase this issue down with no luck. Since the problem started I've cleaned maf twice, tested tps, checked for good spark, checked for vacuum leaks, checked for plugged cat, changed fuel filter, and changed pcv valve and grommet. In the last year ive done Plugs, wires, upstream o2 sensor, coolant temp sensor in back of head, cleaned tb and Iac, and done a comp. Test and got good numbers then. Where my problem seems temp sensitive I ordered another temp sensor to rule out a dud but haven't installed it yet. The hesitation issue seems to be pretty common as there's lots of posts on it but haven't seen any for sure solutions.