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Topic
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To make it straightforward here’s my situation:
Car: 94 corolla 1.8L 7a-fe automatic
DTC: 26 “A/F ratio continuously rich”
Symptoms: rough idling, loss of power, low idle rpm/stalling at idle
Work thus far:
+vacuum at manifold
~18in.hg at idle (no load)
~15in.hg at idle (in gear)
+Plugs look decent, spark-plug tube seals seem to be a little rough as there is oil in the plug-tubes. Started by cleaning the plug wire ends which improved running quite a bit for a few days then symptoms returned seemingly worse/worseningIt is definitely running rich as I can smell gas while running (no line leaks though, I checked)
I can’t seem to find vacuum specs for this car, my vacuum gauge says 18in. is good(green) but 15in. is at the top of the yellow section.
Do i need to look for a vacuum leak or suspect a sensor/vacuum valve somewhere? I read in the chilton that non-CA cars in 1994 didn’t have EGR system.
Thanks in advance for any help. I’ll post sensor resistance/voltage test results when I do them.
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Replies
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Your car is gonna be a bit of a challenge to diagnose because of the year and limited supplied data. I’m assuming that it is fuel injected.
First of all check your coolant level to make sure it’s full.If the coolant sensor is not submerged in coolant it will send the incorrect temperature information to the computer.
Be sure the air filter is clean as well as checking for obstructions in the engine air inlet.
Check the fuel pressure as well as for fuel in the vacuum hose that connects to it. Check the fuel return line for damage.
You will need to get much deeper into it if nothing is found. I think the low vacuum could be caused by running rich.
I will try to find some sensor values for you.
The computer might think it has low vac and increase the fuel. Check the hose to the MAP sensor, it might be leaking or have a faulty sensor. Your vacuum is good and steady, that should eliminate a big vac leak or a valve as the problem. No hissing in the brake booster? Just asking because it is an old car and I have a similar model that leaks a little when the brake is pushed.
Yes it’s efi, since it’s OBD1 there’s no details form when the code was thrown, no CEL. it’s coming on 175k miles and I’m not sure of previous maintenance. I’ll check the coolant and make sure it’s full/bled. It’s a fairly new K&N intake (<10k miles) so air filter is good. doing an intake clean and seafoam to make sure nothing is gummed up from carbon. Also will check the vacuum hose to FPR and fuel pressure if I can (some are easier than others for the tools I have access to. Thanks
The brake booster hose has been cut into by some PO (looks terrible, like they did it with a pocket knife) to install an aftermarket cruise control (which wasn’t even fully connected so I remove it and capped off the T they installed on the brake booster hose. It doesn’t have any clamps on the T though, I’ll definitely try clamping that and checking the MAP hose. Thanks.
Clamped off the brake booster, noticed one of two nipples is broken off of (what i believe is) the “TVV for EVAP” as per label in vacuum diagram, the other nipple has a hose going to the top of the the charcoal canister (which is where one of the TVV hoses goes, the other goes to the TB area which just happens to have an open vacuum tube…hmm)
Recent updated info (ECT sensor checked (~10mins after killing engine on a HOT day=fully warmed up) ~321oms and rising slowly (which according to chart in manual is within specs.
Just checked MAP sensor and voltage to sensor in spec and replaced a section of the vacuum hose connectors (hard plastic vacuum line, connected to nipples at both ends with a section of vacuum hose) with new hose as the one on there was cracked.If everything looks good and tests within specs the next thing i would do is replace the o2 sensor if it appears old.
Warmed up engine, ran for 1200rpm+ for 2+mins, then checked o2 sensor output, it’s a 2-wire sensor and wasn’t sure which was which, one wire gave basically no reading, the other gave a reading of around 0.85v fluctuation with any change in throttle and would sometimes drop to as low as 0.62v. The specs show that it should fluctuate between 0.4 & 0.6v. Is this a bad sensor?
It’s hard to be 100% sure but it appears to be lazy and not in full range. I like to see em switch between .20Mv to .90Mv.
I would replace it for sure.
Tested coil primary and secondary resistance and both fell within specs. Here’s another thing I didn’t think about. The car has a couple of exhaust leaks, the worst of which (judging by where the most smoke came out when we sea-foamed it) Is around the exhaust manifold-cylinder head gasket. Could this leak directly upstream of the o2 sensor cause a false lean reading, thus require the ecu to run a richer fuel map? The leak is pretty bad. Also, would unplugging the sensor and running it for a while hurt anything? would that help isolate the o2s as the problem area? Thanks for the info.
The exhaust leak is a viable theory. You have already determined the o2 is not within range correct, so replacing it seems like the first step.
Also, have you checked fuel pressure, and regulator?
not yet, but I understand that if it’s high, that’s definitely a plausible cause of a continuously rich mixture. The only thing on the o2s is I just realized the computer is holding a DTC26 (continuously rich mixture) which I would think if the exhaust leak is causing the o2s to think the mixture is lean (thus enrich the mixture), that it wouldn’t throw a code saying that it’s too rich… unless the exhaust leak has gotten worse since the code was thrown. I’ll try to get the new exhaust gasket in and new o2s in and go from there. Thanks for the help. It’s MUCH appreciated as this is a daily driver.
[quote=”razer1000″ post=65162]not yet, but I understand that if it’s high, that’s definitely a plausible cause of a continuously rich mixture. The only thing on the o2s is I just realized the computer is holding a DTC26 (continuously rich mixture) which I would think if the exhaust leak is causing the o2s to think the mixture is lean (thus enrich the mixture), that it wouldn’t throw a code saying that it’s too rich… unless the exhaust leak has gotten worse since the code was thrown. I’ll try to get the new exhaust gasket in and new o2s in and go from there. Thanks for the help. It’s MUCH appreciated as this is a daily driver.[/quote]
Don’t remove the o2.The computer won’t know what to do.
Yes the exhaust leak before the o2 will affect its reading.
The leak is causing the oxygen content to be high so the computer
is seeing a lean condition which will tell the computer to richen the mixture.
Fix the leak.Just took it to a reputable mechanic and was told the “num.3 valve compressor” was burned. was only getting 75pounds (this is what my gf told me; I assume they’re referring to the cylinder compression pressure) and was quoted $1100….this is what I was afraid of as I wasn’t able to do a compression test yet. Does that all sound right? Thanks
Also, IYO is it worth it to repair? before this, the car was running/driving/shifting strongly/smoothly. has 172k on it, clean trans fluid, just did timing belt w/p change. With gas costing what it does (ALOT) a 30+mpg car is perfect and the car fits her needs perfectly. A valve change/repair would involve a new head gasket right? In which case the possibility of that going out soon would be eliminated. She’s devastated right now, I am hoping I can help somehow as she’s also been saving for a move soon (was the plan) Thank you very much for the advice and help! 🙂
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