- This topic has 4 replies, 3 voices, and was last updated 10 years ago by
.
- Topic
1998 Toyota Corolla
87,000 miles
1zz-fe 1.8L engine
A131L 3-speed Automatic Trans.Symptoms:
Poor Gas Mileage, heavy drop off in the past 12-18 months. Used to get 24-26mpg during our upper midwest winters, now get 18-21mpg. Summertime was 29-33mpg, now 24-27 mpg.
Went for emissions test. Evap, 02, and catalyst monitors were “not ready”, so it failed. I was expecting Evap and 02 as those never complete, but the catalyst monitor was a surprise. The ECU has NO active codes, nor any pending. Only code I ever get is P0441 for EVAP during summertime (VSV in rear of vehicle is shot, corroded beyond cognition).
Recently I was fooling around with the Torque (android) app and ELM327 and may have cleared the ECU inadvertently. So I wanted to run the proper Toyota drive pattern for emissions readiness (pdf EG003-02). I followed the instructions for the catalyst monitor, but could not get it to go green (using Scan XL Pro). However, the Evap monitor went ready (I think ECU ignores it when ambient temps are below 40F). So I was able to quickly get an emissions test and it did pass (thankfully). FYI, 1996-2000 cars will pass if up to two monitors are “not ready”.
So I just bought myself some time to work on the poor fuel economy issue.
My OBD scanner, Scan XL Pro, seems to show everything is working. No DTCs. Short and Long term fuel trims look good under load (see pic), the LTFE is around +11% at idle (at temperature) which is slightly higher than optimal. The 1998 Corolla doesn’t report O2 sensor voltages, so I can’t tell if the sensors are reporting good data or not. Because the 02 readiness test never completes, the car runs in Open LOOP off its base mappings at all times, as it has for years; years when the mileage was much better.
The catalytic converter is original, both 02 sensors are original. I checked briefly for vacuum leaks with a propane torch and found nothing.
One thing I should mention is fuel mileage seemed to go downhill after I replaced the exhaust “donut” gasket between the exhaust manifold and the pipe that is ahead of the converter (where the upstream 02 sensor is located) I replaced the bolts and springs on the connection and do not think there is an exhaust leak at this location, although there was a leak before the gasket was replaced. Makes no sense.
Any ideas on what I can do, starting with the easy stuff first? Thanks guys.
- Replies
When the ecu does its check on the evap system and it does not pass the check
the emission readiness won’t ready itself.I understand that part. The Evap code is not really an issue that I’m wanting to deal with. The code (P0441) only sets in summertime, and I read elsewhere on a Toyota forum that the ECU ignores the evap system if the ambient temp. at startup is below 40F, or thereabouts. This problem has been active for at least ten years, without noticeably affecting mileage.
I suspect I have a small vacuum leak at idle, maybe due to an aftermarket cruise control unit that was installed. I still feel there has to be something else amiss. The engine is still a baby at less than 90,000 miles, always has oil changes at 2000-3000 miles. As with all 1ZZFE engines, it does burn a little oil, but only 1/4 quart in 3000miles.
My main question centers on the 02 and catalyst monitors always reporting “not ready” and the implications therein on my fuel economy. As best I can tell, the ECU is ALWAYS in open loop. Is this really possible? And if so, what would cause the system to never switch to closed loop? Does it use the O2 sensor data at all in open loop, maybe just for LTFT? I would like to use a digital multimeter to probe the O2 sensor voltages but I need to purchase a new one as well as some t-pins to back-probe the connector. Until then, any other thoughts?
I am not sure why you don’t believe you can get voltages out of the upstream and downstream O2 sensors on a 1998 Corolla (or any OBDII corolla for that matter)… You definitely need to use a different scan tool on this… A harbor freight model will do the job just fine…
What codes are you getting, if any… You should be getting something…
Also, only use genuine Denso O2 sensors on Toyotas, I see you have originals on there…
As for your question about always running in OPEN loop, yes this can occur if O2 data is bad, MAP sensor data is bad, or coolant temperature never gets to the operating temperature.
Check your MAP sensor and ensure it is plugged in, and use a scan tool to check that the MAP is giving inverse readouts relative to barometric pressure when rpms are increased…. I have seen disconnected/bad MAP sensors throw the computers for a loop. and most times it is a vaccum leak or disconnected MAP sensor…
You really should have that EVAP system checked for many other reasons. Just drop the muffler and pull down the canister, and replace the VSV…not a hard job.
-Karl
Thanks for the reply, Karl.
I’ve tried three OBD scanners (2 windows based, plus the Android Torque app), none of them get any data from the O2 sensor PIDs. I don’t know if I want to spend money to buy a dedicated scanner with live data at this time. I’ll probably try to probe the upstream O2 sensor voltages first. I also don’t want to buy new Denso sensors until I have a valid reason to do so. I have seen another forum post or two that said the 1998 ECU doesn’t support the O2 sensor voltage PIDs, don’t know if this is true but it’s starting to seem that way. The 1998 Corolla was the first year of Gen 8, its possible Toyota refined the ECU in later years.
I absolutely have no DTCs. Only codes I’ve seen over the last decade have been EVAP (p0441/p0446).
I will look at the MAP sensor. I do get data from it (see attached pic in first post) but I don’t know if the data is valid.
The one question I have is if the ECU is stuck in open loop, how are long and short term fuel trims being computed, or are they all just derived from stored data arrays without any input from vehicle sensors?
As for the EVAP VSV, I’d be inclined to look into that issue after I solve or improve the fuel economy. I found an aftermarket VSV on ebay for $25, which would be worth trying.
- You must be logged in to reply to this topic.