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2003 Discovery S
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Discussion Starter #1 (Edited)
Hi,
First some recent history.
I just changed the intake manifold gasket on my 2003 Discovery with a Bosch V8, 125K miles. I changed because of a coolant leak. That was noticed after the water pump failed, and overheated. After I finished those repair, I had a misfire on cylinder one code.

I replaced the spark plugs, and noticed that they were very clean, just a very small amount of brown. Now the new plugs, after may be 40 miles, are white, no brown seen on one that I pulled. So, it is running lean. no codes.

I sprayed all around the intake manifold with propane, and did not detect any leaks. I reset the adaptations, so the fuel trim values went to 1.00, still white plugs.

When I watch the O2 sensors on my scanner, the voltage swings between .05 and .75 volts, starting at about .45v. I tried introducing a leak, and a rich condition with propane, so I could watch the sensors, but I killed the engine each time. I will try again later.

At idle the MAF is measuring 24Kg/hr, and increases with rpm. The air temperature sensor seems to be reading a little high, but, I will check that in the morning when the engine is cold, to get a better measurement.

So, How do I know when it is dangerously lean? Any tips on troubleshooting?
Thanks,
Steve
 

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Super Moderator
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2,118 Posts
Welcome to the forum.
MAF sounds about right.
Are you currently setting any fault codes?
What engine temperatures are you getting and what were you getting before the repairs?
It sounds like you do have some general knowledge and experience, how confident are you on your evaluation of the plugs being “ white”?
 

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2003 Discovery S
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Discussion Starter #3
Thank you.

No codes. Just checked the coolant and air temperature sensors, first thing in the morning, and got 55F and 57F. When running it runs around 190F. I will check that again later. I don't know what the temperature was before repairs. So they are close to each other and the ambient temperature.

I will post a picture of a plug a little later this morning. I did not save the old plugs. Initially when I pulled the old plugs, with about 50k miles on them, the rear plugs had a little more brown than the front. The front plugs just had a little brown around the electrode, the sides of the insulators were white. I'm more familiar with looking at plugs on a carbureted engine. I am using Bosch platinum plugs. They look unused at the moment. Do these engine normally run on the lean side? When tuning a carburetor, I just drive around the block, and I can see color on the plug. The engine runs well, no drivability problems, although I think I'm hearing the tapping of a loose liner.

If the O2 sensors are coated, or contaminated, what direction would that shift the mixture? Would it not see all the O2, think it's rich, and lean the mix?

Thanks,
Steve
 

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2003 Discovery S
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Discussion Starter #4
Here is a plug.
 

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2003 Discovery S
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Discussion Starter #5 (Edited)
I just sprayed a can of carb cleaner all over the intake manifold, at least everything I could reach, all the vacuum hoses, the plastic intake hose connecting the MAF, the brake booster, everything I could think of. No change in RPM. I disconnected the small vacuum line, and the O2 sensors responded low, and sprayed carb cleaner in that port, and they went high. They seem to be working. The O2 sensors have about 40k miles on then, changed them in 2011, The coolant temperature is 190 when warmed up. The tempreture gauge was alwas in the middle of the scale, before and after repairs, except for the overheating incident.
 

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2003 Discovery S
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Discussion Starter #6
Looking at the picture of my plug. I noticed the part of the plug around the threads are darker. So, may be just the heat range of the plug? May be nothing to worry about?
 

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very disco
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1,182 Posts
FYI the temp gauge is rubbish. It has three positions: low, mid, overheat.

do you have a stoichiometric gauge? Stoichiometer? I effing hate that word.
 

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2003 Discovery S
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Discussion Starter #8
No stoichiometric gauge . I'm getting temperature measurements from the sensors through my scanner. I was just referencing the dash gauge because I did not have the scanner hooked up before the problems began.
 

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Drives like an old man
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398 Posts
I'm guessing here, so feel free to disregard. Since you had 50,000 miles on the old plugs, is it possible the engine was compensating for fouled plugs and now needs to be reset to relearn the correct mixture? As I said, just a guess. Sounds like it's running well. May be there is no problem.
 

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Drives like an old man
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398 Posts
I'm guessing here, so feel free to disregard. Since you had 50,000 miles on the old plugs, is it possible the engine was compensating for fouled plugs and now needs to be reset to relearn the correct mixture? As I said, just a guess. Sounds like it's running well. May be there is no problem.
 

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2003 Discovery S
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Discussion Starter #11
Thank you. I did reset the adaptations, and the old plugs looked just as clean.
 

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2003 Discovery S
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Discussion Starter #14
Checked a couple of plugs today, after a drive, and there was some black on the insulator. The electrode tip good. I guess it's OK. I will just drive it. I just didn't want to burn up anything. Thanks.
 

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very disco
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1,182 Posts
Browning is normal, but it shouldn’t be turning black?
 

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If you’re not getting any codes or engine warning lights and temperatures are in a normal range, I would drive it and not worry too much.
You’re sure the plugs are correct for the vehicle??
 

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2003 Discovery S
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Discussion Starter #17
I will look at them again next week to see how they look.I had a very small brown spot that vanished. I think I.m looking at them too much. I had the plugs on my parts shelf for a couple of years, I wrote "Rover " on them, so they should be correct. I will double check the part number tomorrow. I have a bad habit of buying parts, and not getting around to installing them for years. I will drive it for a while and check them next week.
 

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very disco
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Haha yeah. Maybe you’re all in your head about something when it’s nothing. I’m guilty of that sometimes.
 
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