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FFCobra Captain
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Discussion Starter · #1 ·
Because of the blown head gasket, I know I was spewing a lot of coolant into the exhaust. I found out today that could cause the o2 sensor to go bad. I'm going to replace that sensor.

Question is as follows:

When driving the car today, when I got on it hard...at around 4500rpm it would sound like it was stubling. I'm not sure if that's the sound of detonation or just the engine missing. Obviously, I don't want to chance it, so I didn't push it harder.

Could a bad O2 sensor cause that sympton?

When I take the O2 sensor out to replace it, is there anything I should look at on the sensor to tell me if it's fried?
 

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Dan
Yes a bad O2 sensor could cause this. I would think that you are hearing a lean stumble because it is likely that the O2 sensor is needed to richen up the mixture to match the extra air provided by the SC.

There is nothing normally visible to indicate a bad O2 sensor unless it has broken or burned wires on the pigtale

DD
 

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Premium Member
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If you are using an EEC-IV ECU, it doesn't read the o2 sensors at wide open throttle..

Make sure you have properly spoofed the EGR sensor or have it all hooked up. This alone will cause detonation as the ECU will keep leaning things out because he's not getting the right EGR feedback.
 

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FFCobra Captain
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Discussion Starter · #4 ·
My computer is an A9P. I don't think it's the EGR.

I have the right resistors in there to spoof the ECU and the car was running fine before I blew the head gasket.

I don't think the car was at WOT, but maybe close.

I'm asking about the O2 sensor because I was told they are sensative and that if coolant hits at while it's hot, it could cause it to fail.

Does detonation sound or feel like the car hitting a rev limiter? I had the dyno-tuned early this year, so I know my fuel system is good.
 

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Hi Dan,

Typically detonation does not cause the engine to act like it's misfiring. It will, however, feel down on power as the detonating cylinder(s) are fighting against the inertia of those cylinders that are contributing power..

WOT is determined by the TPS. You'll usually see maximum voltage just shy of the WOT position of the throttle.

Could the o2 sensors be damaged? Possibly, but you'd probably get a code set and the check engine light would be on.
There is no way to really test an o2 sensor reliably. Do not be tempted to try and ohm one out with a Volt-Ohm meter as that passes current through the o2 sensor and will damage it.

I have had pretty good luck in removing the o2 sensor and cleaning them with a propane torch to burn-off the residue from the sensor. After it cools you can tap it lightly on a wooden block to shake the crud loose from the sensor..

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Fuel system
You may have a restriction in your fuel filter that is causing reduced flow/pressure in the higher RPMs and loads. I would swap the filter.
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I hope this helps..
 

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613 Posts
If the headgasket blew and you got coolant in the exhaust it coated the oxygen sensor. Antifreeze has a silicone compound that will ruin an O2 sensor. Best to replace it anyway. That might cure your problem. mike
 

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If the O2 sensor is dead, ie not switching, then it will throw an error code and the MIL light will come on. The stock A9L(P) program goes open loop at a TPS voltage of 2.7v or slightly more than 1/2 throttle. At WOT the EEC is not looking at the O2 sensors to set the fuel mixture only the WOT fuel tables. If it's only acting up at WOT then I don't think it's an O2 sensor.
 

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FFCobra Captain
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Discussion Starter · #8 ·
If it's not the O2 sensor, then what should I be checking? Should I go get a new TFI module for the distributor to see if that solves the problem? I think the one I have is from the donor car.
 

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Originally posted by Dan Babb:
If it's not the O2 sensor, then what should I be checking? Should I go get a new TFI module for the distributor to see if that solves the problem? I think the one I have is from the donor car.
Originally posted by Big-Foot earlier:

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Fuel system
You may have a restriction in your fuel filter that is causing reduced flow/pressure in the higher RPMs and loads. I would swap the filter.
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section 8
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Regardless of anything else whenever we replace a head gasket we always replace the o2 sensors. did you reset the timing correctly ? Bob
 

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FFR Master Builder
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Dan, I dont want to highjack your thread here but there is some good info that explains what is going on with my car. I am supercharged also with 12lbs and I have been having some problems lately. I have been supercharged for about 3 years with no problems until about 4 months ago. I did pull the EGR off this year but never put in the resistor. It has sounded like I have been hitting the rev. limiter also on hard WOT. I pulled the plugs and they were white so I knew something was up but found it hard to believe the fuel system was not up to the challage anymore. I for saftey sake put in a FMU and it didnt help. What type of resistor do you put in and on what wires?
 

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FFCobra Captain
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Discussion Starter · #12 ·
Randy...I'll get the fuel filter today and also pick up a new O2 sensor for that one side just to be safe. I'll also take off the dist. cap to see if any of the coolant that spewed out when the HG blew foulded up the inner workings.

Bob...I didn't pull out the distributor, why would I have to reset timing?

John...I used the instructions I found here on the forum. I think they are based on the info found at fordfuelinjection.com. If you do a search, you may find them...or you can just buy the getup from that site.

I really think this new issue has to be related to the HG blowing. The ran absolutely perfect for a day & a half of pounding at the track. It seems to be too much of a coincidence that now this problem has popped up.
 

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perhaps this issue was the 'cause' of the head gasket blowing...not the other way around. If it's lack of fuel, that would certainly explain the head gasket!
 

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What's your fuel pressure under boost? Fuel pumps do get weak with age and it will show up more when the FPR (or FMU) is raising the fuel pressure under boost. The flow rating of the typical Walbro pump drops by 10-20% for each 10 psi of fuel pressure increase. So at 10 psi of boost (50 psi fuel pressure) a 255lph pump may only supply a little more than 200lph. If you have an FMU that raises the pressure to 60 psi + than the flow rating will drop to well under 200lph. This can easily cause a lean condition and result in blown head gaskets or worse.
 

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Dan:

With the boost you have I would recommend getting a fuel pressure gauge and a wideband O2 sensor that you can read on the dash. I have both and they can save you a lot of grief. With the wideband you can see when the car is leaning out when it should not and that can save you more blown head gaskets and damaged pistons... Hearing the detonation is near impossible with all the other racket going on.
 

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FFCobra Captain
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Discussion Starter · #16 ·
I have 30lb injectors with a matching MAF and a 190lph pump. The pump was new when I built the car.

The head gaskets were the original ones from the donor. I had the car dyno tuned earlier this year and the A/F ratio was good all the way to over 6000, so I really think that part's ok.

Good suggestion though Shep.
 

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Dan,

Simple test, just pull the leads off of the O2 sensors and the EFI will revert to 12:1 A/F ratio on it's own. You will get a check engine light after around 5 min of driving, but it is a "soft" code and you can drive your car forever that way.

BTW, it's not a bad way to run a SC motor... much safer then 14:1 under full boost.

Let us know how it works out..
 

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Dan

If you don't know that your A:F ratio is good don't drive the car under boost until you get it checked. Minimum you should confirm that the fuel pressure is what it should be under boost. Unless of course you like changing head gaskets. :D

I chased a problem when I had the KB on my Mustang for about a year and went through 3 sets of head gaskets before I finally narrowed it down to the fact that the fuel pump had gone weak. The pump was brand new when I put the blower on the car. It wasn't until I added a fuel pressure gauge that it became obvious that I had a fuel delivery problem. By the way a 190 lph pump on you motor is marginal at best. 440 ftlbs at the wheels is well over 500ftlbs at the crank. That needs a LOT of fuel.
 
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