O2 Sensor & Poor Cold Idle Issues

I have an ’87 B2 that has the EGR system plugged, and a few of the extraneous vacuum system lines also plugged/disconnected. I disconnected my O2 sensor some time ago due to a severe cyclic surging/hesitation problem, and the engine has run well ever since (in summer). I currently get about 16 mpg (summer/carefully measured), and except for cold starts, I'm reasonably pleased with how the engine runs.

I have two questions/issues. I have a significant exhaust leak where the exhaust manifolds join the Y-pipe (both sides, but worse on the passenger/O2 sensor side). The Y-pipe itself looks OK, just that the flange has rusted away and is no longer holding. I have purchased one of those split flanges, and will eventually seal this exhaust leak up, but I simply do not have the time to do this (I anticipate an entire day or more to do this, due to the very limited access to this flange). I have a replacement O2 sensor, but am hesitant to install it as I do not know if O2 entering from the manifold/Y-pipe exhaust leak might screw up readings and/or “burn out” the new unit (cost >$80). So, after reading the tech pages, I see I’m likely running “open loop” on the O2 sensor, and therefore with too much fuel to the engine.

Question 1: Will installing and connecting the new O2 sensor while I have a significant upstream exhaust leak cause either poor performance or sensor damage? (Been running with the bad O2 sensor simply unplugged.)

Also, while the engine has always been a GREAT starter, with the onset of cold weather, it will no longer stay running and idle when cold. This is a pain-in-the-ass, as I can’t let the vehicle run for a few minutes to warm it up before driving, and it will stall out numerous times if I try to drive before it is warm. I would think that the open loop O2 sensor operation would help with this, but it doesn’t seem to. Once warmed up, it runs fine. (I have the idle set with the idle set screw at about 1400 rpm when warm to help with this problem.)

Questions 2: Why won’t the motor idle properly when cold? (I have a new temperature gauge temperature sensor, so this is working well.) What can I do to restore cold start idle? Is there some method to “fool” the fuel system into enriching the mix while it warms up? Perhaps shorting something out with a dash mounted “choke switch”?

BTW, since I registered the B2 as a “historic” vehicle, I no longer have to pass MD emissions testing, so this is not a concern. Timing is set, plugs are good etc. etc.. (I’m also interested in the 88 TFI replacement modification mentioned in the “2.9 EGR” thread, but need to take care of my O2 problem first.)

Any help/insight would be appreciated. Thank you.

CraigK

CraigK said:

...Question 1: Will installing and connecting the new O2 sensor while I have a significant upstream exhaust leak cause either poor performance or sensor damage? (Been running with the bad O2 sensor simply unplugged.)

Also, while the engine has always been a GREAT starter, with the onset of cold weather, it will no longer stay running and idle when cold. This is a pain-in-the-ass, as I can’t let the vehicle run for a few minutes to warm it up before driving, and it will stall out numerous times if I try to drive before it is warm. I would think that the open loop O2 sensor operation would help with this, but it doesn’t seem to. Once warmed up, it runs fine. (I have the idle set with the idle set screw at about 1400 rpm when warm to help with this problem.)

Questions 2: Why won’t the motor idle properly when cold? (I have a new temperature gauge temperature sensor, so this is working well.) What can I do to restore cold start idle? Is there some method to “fool” the fuel system into enriching the mix while it warms up? Perhaps shorting something out with a dash mounted “choke switch”? ...

CraigK

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If you have significant exhaust leaks the engine is going to run like ass once it is warm, the O2 sensor will trick the computer into thinking that the engine is running lean and richen it up. The oxygen sensor function has no bearing on how the engine runs at a cold start. The guage sensor for engine temp and the computer sensor are two different things. the sender for the computer is hidden behind the thermostat housing. the guage sensor does not interface with the computer at all. The TFI module is the grey box on the back of the distributor. It has no bearing on the computer timing it just reads the hall effect sensor and fires the coil from the timing curve that the computer feeds it. The EEC is what is different between the 86-87 trucks and the 88-92 trucks. This is the mod that was being discussed. This is behind the passenger side kick panel in the cab. Without pulling codes (procedure described in tech library) you are really just shooting in the dark as to your cold idle issues.

holyford86

I went out and looked, and there is another sensor behind the thermostat housing and slightly to the left (looking at the motor from the front of the vehicle). Much bigger than the smaller gauge sensor which located slightly to the right. I guess this is the temperature sensor that signals the EEC???

You indicate the cold idle problems are not due to the O2 sensor (thanks). The (bad) O2 sensor is disconnected now, running "open loop" with maximum fuel?, so installing a new one wouldn't really richen it up, would it? The vehicle runs pretty well now once warmed up.

Thanks for the information, and for clearing up my TFI/EEC confusion.

CraigK

No one has yet satisfactorily explained to me how an exhaust leak effects the O2 sensor. Manifolds are under pressure, and outside air cannot enter.

I currently have a 1996 F150(OBDII) that has a leaking exhaust gasket at the cylinder head now for about 6 months, and no check engine light. I also have a 1988 Merkur (OBDI) that has developed a substantial leak at the turbo flange, and neither vehicle is affected by the leak.

You say your truck is leaking at the flanges, which is past the O2 sensor anyway.

The engine coolant temperature sensor works as the automatic choke on EFI vehicles. The computer checks the water temp, and determines if the engine needs more fuel at start up due to being cold, or less if the engine is warm.

Another thing that effects starting is vacuum leaks. This will cause the engine to start, then die, repeating until the engine is somewhat warm. Usually, you will have a faster idle with the leak.

With the O2 unplugged, and any other modifications you have done to the emissions system, the computer is going to run on pre-sets which will make it run richer than normal. This is what would damage the O2 sensor.

Twisting on the idle screw will also upset the idle circuit, and can cause a richer condition. shady

shadetree said:

No one has yet satisfactorily explained to me how an exhaust leak effects the O2 sensor. Manifolds are under pressure, and outside air cannot enter.

I currently have a 1996 F150(OBDII) that has a leaking exhaust gasket at the cylinder head now for about 6 months, and no check engine light. I also have a 1988 Merkur (OBDI) that has developed a substantial leak at the turbo flange, and neither vehicle is affected by the leak.

You say your truck is leaking at the flanges, which is past the O2 sensor anyway.

The engine coolant temperature sensor works as the automatic choke on EFI vehicles. The computer checks the water temp, and determines if the engine needs more fuel at start up due to being cold, or less if the engine is warm.

Another thing that effects starting is vacuum leaks. This will cause the engine to start, then die, repeating until the engine is somewhat warm. Usually, you will have a faster idle with the leak.

With the O2 unplugged, and any other modifications you have done to the emissions system, the computer is going to run on pre-sets which will make it run richer than normal. This is what would damage the O2 sensor.

Twisting on the idle screw will also upset the idle circuit, and can cause a richer condition. shady

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I didn't realize thats how it was with exhaust leaks, although it makes perfect sense. No, the manifold flanges are not past the O2 sensor on a 2.9 the O2 sensor is where the Y pipe comes together at the cat.

The "idle screw" on a 2.9 isnt for setting the idle speed, the computer does that, its for setting the throttle position sensor base voltage.

holyford86 said:

No, the manifold flanges are not past the O2 sensor on a 2.9 the O2 sensor is where the Y pipe comes together at the cat.

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The system is still under pressure, and shouldn't effect O2 readings.

The "idle screw" on a 2.9 isnt for setting the idle speed, the computer does that, its for setting the throttle position sensor base voltage.

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It also will change idle speed. The computer only controls the positon of the idle air valve. If you allow more air past the throttle plates, idle will increase, as Craig indicated in his original post. shady

shadetree said:

The system is still under pressure, and shouldn't effect O2 readings.

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If it's leaking before the o2, the readings are going to be slightly off. Exhaust is leaking before the o2 sensor, I don't how you could interpet that any other way.

shadetree said:

It also will change idle speed. The computer only controls the positon of the idle air valve. If you allow more air past the throttle plates, idle will increase, as Craig indicated in his original post. shady

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But shouldn't be adjusted from the factory, or to band-aid an idling problem.

The 3 wire temperature sensor on the left side of the intake near the alternator is the PCM sensor, the single wired on the right is the gauge sender.

Pete

The exhuast stream may seem like a continuous flow, but it's not; it's pulses (that are taken advantage of with headers and strategic cam timing).


Those pulses could very easily pull in O2 from an opening.

almostclueless said:

The exhuast stream may seem like a continuous flow, but it's not; it's pulses (that are taken advantage of with headers and strategic cam timing).


Those pulses could very easily pull in O2 from an opening.

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THANK YOU, that is the correct answer. Either way it does affect the engine's running condition, mileage and power.

Adjusting the idle with the idle screw is a no, no. Correct, it covers up a problem, but it does adjust idle and is not there just to adjust voltage.

Exhaust pulses are separate on each cylinder. When they are combined in a manifold/header, the flow is pretty much constant on multi-cylinder engines. The manifold is under pressure. This is one of the reasons why an air pump is used in emissions control.

Camshaft overlap is designed to enhance flow OUT of the system. Reversion occurs mostly from backpressure. shady

Natedog said:

THANK YOU, that is the correct answer. Either way it does affect the engine's running condition, mileage and power.

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Really? When will I see the effects of my two vehicles that have leaking exhausts now for quite some time???? shady

Instant Fix!

Thanks everyone, especially holyford86. I replaced the engine temperature sending unit, the one that signals the computer, and also a bad coolant thermostat (stuck open 195° unit out, new 180° unit in) and my cold start idle and engine performance improved instantly. The engine also seems to be more responsive at all speeds, even after warm-up. I was also able to back off the throttle stop screw a quarter turn.

I'm wondering if this simple change will be evident in gas mileage, as the engine was probably running lean with the bad temperature sensor(???), especially when cold. During the last mileage check, (each ~500 miles I fill the tank to absolutely full and check fuel consumption against miles), mileage had dropped from 16.1 mpg (two previous checks) to 14.9 mpg, but this most recent 500 miles included much colder weather, cold starts and some 4x4 use in the snow.

I will run another 500 miles or so to get a new baseline mileage reading with the new temperature sensor, then try installing the new O2 sensor (with the existing exhaust leaks), just to check the effects.

CraigK

PS The bad news is that I seem to have a leak in my heater core. Windshield is fogging up. If it's not one thing, it's another.

I would install a 195* thermostat in place of that 180 unit, that might be why your gas mileage is in the toilet. The heater core takes all of about 20 minutes to change in these trucks so its really not a big deal.

shadetree said:

Adjusting the idle with the idle screw is a no, no. Correct, it covers up a problem, but it does adjust idle and is not there just to adjust voltage.

Exhaust pulses are separate on each cylinder. When they are combined in a manifold/header, the flow is pretty much constant on multi-cylinder engines. The manifold is under pressure. This is one of the reasons why an air pump is used in emissions control.

Camshaft overlap is designed to enhance flow OUT of the system. Reversion occurs mostly from backpressure. shady

Really? When will I see the effects of my two vehicles that have leaking exhausts now for quite some time???? shady

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Dude how much does everyone have to fucking argue about this with you. I swear this is the only type of goddamn topic you ever post in.Ever seen a one of them cheap sandblaster? The air is under pressure, the air is rushing past the a hole which leads to the sand supply and sucks up sand with it and shoots sand out of the nozzle, It's called the venturi effect.

Hahnsb2 said:

Dude how much does everyone have to fucking argue about this with you. I swear this is the only type of goddamn topic you ever post in.Ever seen a one of them cheap sandblaster? The air is under pressure, the air is rushing past the a hole which leads to the sand supply and sucks up sand with it and shoots sand out of the nozzle, It's called the venturi effect.

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Getting a little testy there aren't we Hahns? And, you don't have to argue about it at all, just move on, no one is holding a gun to your head. You don't get around much if you haven't seen my posts in other areas.

I'm not talking about a sandblaster. I know that when air is moved it drags a lot of air with it. Some systems are designed to use this additional air flow, but this is an entirely different situation.

I have worked on hundreds of these systems, and have never seen a problem with small leaks. Some manifolds crack quickly without detrimental affects. Go find a manifold that is not cracked for a 92 Toyota Corolla 4 cylinder, or Chevy Prism, and other such models. Kind of hard to find, and the cars run fine, but the noise drives you nuts. Also, these vehicles still pass Texas state emissions inspection which is one of the more stringent.

As I stated before, two of my vehicles currently have exhaust leaks, and have given me no poblems.

Take a chill pill dude, life is too short, and at 19 years old, wait until you have a little experience with this sort of thing before trying to give me a dressing down. shady

holyford86

The engine, once warmed up, seems to run just fine. Both before and after replacing the temperature sensor. (Okay, perhaps a bit better after.) Mileage is probably not too bad (not atypical for B2's) considering it's an AT, the last 500 miles included some cold weather starts (used lots of gas pedal to keep the engine running when cold, and idle was set high) and some 4x4 use in snow, and the O2 sensor is still disconnected.

I went with the lower temperature thermostat out of concern of cracking the heads, which seems to be a common problem with these engines. I just thought running a few degrees cooler would keep the engine further away from that point. There was a 160° thermostat available also.

I'll try to get to the heater core this week, it does look fairly easy, and Autozone has a unit in stock for $22.

CraigK