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I have a 2004 Toyota Echo. It has been running pretty bad at idle, I don't have the CEL on. I scanned it and when warmed up the LTFT are up to 32%. When I rev it up to 2000-3000 rpm they will instantly go down to about -3%. I have been suspecting a vacuum leak but tested every aspect of it with brake cleaner, propane and even built a smoke machine to check for leaks. I haven't found any. The car drives pretty good and the rough idle symptoms are gone when you rev the engine slightly higher than idle. The car runs really good when I cold start it and usually will take a good 15 min for the LTFT to gradually increase to 32% when everything is warmed up and the car is idling at around 650-700 rpm.
I understand to help diagnose a vacuum leak with the LTFT you rev it and see if they go down gradually, mine drops instantly. Readings from the upstream o2 sensor looks ok, jumps around from 0.05v to 0.9v. I replaced the downstream o2 recently because it was producing over 1v and lit up the CEL. The reading are a bit off for this one, on idle it will stay under 0.1v, even as low as 0.035v, but if giving a quick snap of the throttle will go up to 0.8v. I have been suspecting the downstream o2 of being the cause of this issue but read that the rear o2 is not used for fuel trim but only to monitor the cat efficiency.
The car has a small exhaust leak at the manifold gasket, but it's been like this since I had it and they was no problems then. I am pulling my hair with this one. The only aspect I haven't checked yet is injector related.
I find it really weird that the CEL haven't came on yet, and the fact that the LTFT are super fast changing after a quick rev is puzzling me.
It still sounds like a vacuum leak to me, or else that manifold leak is causing trouble now.
If possible I'd try to fix that exhaust leak to eliminate the variable, but you can also try pinching off the brake booster hose and the purge valve hose and see if you see a drop in STFT. Those two can hide from the vacuum tests you have performed.
I tried to pinch both brake booster and evap hose, didn't change a thing. I replace throttle body and IAC gaskets, I cannot see the IAC position but the idle seems to behave normally on a cold start, going dowhill from 1500ish to 700ish rpm over a 10-15 min window. I don't know the fuel pressure, but it starts without issue and there's no hesitation when driving. I cleaned the MAF with CRC and it behaves normally.
The LTFT drops pretty much instantly when the rpm are increased by let's say 200-300rpm. It goes from 32% to -3% in a blink of eye and goes back to 32% the second I let it go back to idle. When I start it both STFT and LTFT are at zero or slight negatives and then slowly creep up to 32%.
The rear o2 that I replaced was not with a Toyota part but with a Denso that is supposed to be the equivalent. The rear o2 reads extremely low at average 0.04v on idle then when I go down the road at 50 kph, it stays the same then shoots to 0.8v, then back down, it does not seems to stay stable.
taking 15min to get the 02's moving is way too long... should only take a min or two... sounds like the heater circuit is not working. you need to graph those upstream 02's and watch them come to life. voltage should start high then drop within a few min then start oscillating 100 to 900mv. if they fix lean (low voltage) hold the RPM steady at 2500 to 3000 and see if the 02's start moving. if they move then return to low voltage when the RPM drops to idle, the heater circuit is not working.
toyota sets codes at 35
"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does
Last edit: 5 years 7 months ago by VegasJAK. Reason: Added more info
cant it be, that the fast change he is talking about , is just moving into a different fuel map cell? one that has the 32% LTFT's learned for that rpm/load cell, and that its just moving in and out of it?
I checked the heater circuit resistance and it's ok. The voltage is not stuck lean, it will go up in the 0.8v range after a quick throttle snap. I went to the junkyard to get 2 downstream o2 and made sure they were Toyota parts. I will have to try one out just out of curiosity.
Good point about the fuel trim cell, for now it's the only way I can try to explain what I'm seeing.
IAC and MAF are good, I cleaned both and readings are good. I took a compression test just to make sure everything was fine mechanically, all cylinders scored around 205 so it's all good on this side. I replaced the new Denso rear o2 by a Toyota one I got at the scrapyard, it didn't fix it. I disconnected the battery ground to reset LTFT memory even after testing it for a long time at idle and it still looks the same.
I took off the intake manifold to look at the gasket even though I tested it with propane, brake cleaner, water and smoke test because I just don't know where to look from now and I figured it was easy enough just to have a look. Gasket don't look too bad but on cyl. 3 look like some dirt/corrosion might have got past the gasket, it looks a bit shady. I bought a replacement gasket which I should get early this next week. I'll keep you updated.
the IAC ( Idle Air Control motor&valve) and the IAT (Intake Air Temp sensor) are two different things... IAC controls air intake past the throttle plate at idle whereas the IAT is a sensor that measures the temp/density of air drawn in. the MAF resistors are deep inside the air tube of the MAF unit while the IAT is usually visible along side the air tube.
the MAF is non moving mechanically, it reports air flow and voltage so i'm not sure what you mean that its behaving normally. MAF g/s for your engine should be around 1-2g/s at idle.
"an open mind let's knowledge flow in and wisdom flow out for a man who has neither never listens to those who have both".
Being wrong doesn't bother me, it's being right and not understanding why that does
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