What would happen with engine if o2 sensors were unplugged?

[mattd1979]

I have a buddy of mine that recently got a 1990 Q45 that belonged to my Nephew. The problem is that the car wants to stumble and hesitate when it is idling and when he goes to accelerate, it sounds like only have the engine is running until he gets it up to speed. I hooked my analyzer up to it and it noticed that both o2 sensors would read rich at idle and then lean at the upper rpm range. And other times it would read lean at idle and rich up top. There was also a time where one o2 steadily read lean while the other read rich. I wondered if maybe they were going bad. I checked the MAF sensor and the TPS and both were in range as per the shop manual. Unless there is a problem with the fuel pressure regulator, I don't know what else it could be. The reason I asked the question about the o2 sensors is because I wanted to try a little experiment on my 1990 Q to see if it would start to show similar signs.

Thanks

MattZephyrhills, FL

[Q451990]

Have you ohmed the injectors yet? Those O2 sensors sound shot, but it's possible that the engine really is running lean if a lot of the injectors are seriously fubared.

Having a scanner and one good Q beside one bad one is an ideal situation!

As far as how the car would behave with unplugged sensors, I don't know. I know Q45Tech once mentioned that sluggish sensors could cause the car to run rich enough to melt the pre-cats.

Heath

[Q45tech]

Don't confuse 90-93Q with the redesigned cats in 94-95 or the further redesigned 96.

[mattd1979]

Speaking of 95, the engine that is in the 90, I swapped from a 95 which had around 62k on it. The engine if revved up a couple of times will clear up for about 30 seconds and then start back again. As far as ohming them, I don't recall if I did.

Shortly after I first posted, My nephew used my car to go up to a store and it cut off on him 3 times like turning the key off so he called me and as he was describing what it was doing, it started to get worse and towards the end he could barely keep it running. When he revved it up it wouldn't go past 6500rpm and the car has a NICO ecu installed on level 3. A couple of days before he drove it until it was right at empty and it started surging and stalling on the way to the house so I figured he ran it out. So I went up the road and filled a 5 gallon can with BP 93 and added a bit of ISOHEET as I always do when I fill the tank and the rest of that day it drove fine until today. He said that it is almost acting like the other Q now.I replaced 2 injectors a week ago and replaced the fuel filter and changed the oil in my Q. All injectors ohmed at 13 except for one that was at 15 ohms. The car idles smooth as glass.

[sdkhalsa]

In addition to ohming the injectors and oxygen sensor, better check the knock sensors. Also you need to tee-in a fuel pressure gauge and a vacuum gauge. A quick check of the IAC(probably needs to be cleaned) and EGR are also in order. Given the mileage of the vehicle, and assuming minimum amounts of maintenance, you probably have a few things going on at once.

[mattd1979]

The engine has about 68,000 miles on it now. It came out of a 95 Q. Where would be the best place to tee in the vacuum gauge?

[sdkhalsa]

The engine has about 68,000 miles on it now. It came out of a 95 Q. Where would be the best place to tee in the vacuum gauge?

My choice would be the constant vacuum line at the top of the carbon canister. This is not the line entering the canister from from the canister control solenoid. What I did was to get a long section of vacuum line so that I could place the gauge in front of the windshield, behind the windshield wiper. This allowed me to drive around and see what was going on under different running situations. I did the same with a fuel gauge. A couple of obvious places for a leak are stuck IAC and EGR valves.

[Q451990]

I typically tee in at the fuel pressure regulator.

Heath

[mattd1979]

This is an update pertaining to the 90 Q that now belongs to my Mom. I unplugged the o2 sensors and the engine stopped it's hesitation and stumbling.

[Q451990]

I believe it will lean the engine out... there's a TSB for cleaning the spark plugs that involves disconnecting the O2 sensors with the idea that it will heat the plugs up.

I'm going to guess that your 02 sensors were bad and causing a rich running condition - but that's a wild guess on my part.

I would certainly replace them and see what happens - it's time just based on normal maintenence. Even if it doesn't cure your problem, you can eliminate them as the cause.

Heath

[mattd1979]

Yeah you guessed right. The sensors are bad. I verified it when I hooked up my laptop to the car and monitored the sensors. They would both flip back and forth like they are supposed to and after about 5 seconds of running or more, the needles on both gauges would flutter for a second and then peg to the rich side.