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Old 11-12-2007, 07:22 AM   #1
djmcmath
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Location: Rockville, MD
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Engine codes: P1620, P0128, and P0174? Related?

My car has thrown 3 codes all at once, and I'm trying to figure out if they're related before I start replacing sensors in random spots in the engine bay. The car is a 2001 330i that I bought about a year and a half ago, currently about 120Kmi on it.

P1620 - MAP Cooling thermostat control circuit signal high -- from my research here on this board, I think that's pointing to the thermostat for the cooling system, which sits on the very front of the engine, at the top, in it's neat little housing. I've taken a couple of resistance readings on the pins, however, and it seems to be normal: 4-10ish ohms, depending on how long it's been since I've driven. What else would cause that, and can I do more checks without replacing anything?

P0128 - Coolant thermostat (Coolant temperature below t-stat regulating temp) -- again, the search feature here points to a bunch of things, but the thermostat seems to be the most common. Sadly, I think my t-stat is working fine. One thread mentioned another coolant sensor on the side of the engine under the intake manifold -- is that a t-stat like the one on the front of the engine? Can I test it like I did the other one? What's it's actual name, so I can order the part? What else would cause both of these?

--Possibly related issue: I get the yellow-caution coolant light fairly regularly. When it happens, I wait until the car is cold, then add a tablespoon or so to the coolant system -- it's usually only the tiniest bit down from full. There is usually white stuff in the vicinity of the coolant fill cap, as if some coolant has over-flowed and boiled off, oddly enough. Related? Just a weird coincidence?

P0174 -- System too lean, Bank 2. I think this one is unrelated to the other problems, but I thought I'd mention it here just in case. My process is going to be:
1 - Clean the MAF.
2 - Check for dirty fuel filter.
3 - Replace O2 sensor, bank 2.
4 - Replace O2 sensor, pre-cat.
-- Does that sound right?


Thanks in advance, everyone.

Dan
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Old 11-12-2007, 09:54 AM   #2
Chris C
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Looks like your thermostat is shot. I'd replace it. If the thermostat isn't able to regulate the engine's proper operating temperature, then that may cause other trouble conditions like your system too lean reading. You might as well replace your waterpump if it hasn't been replaced before. Folks tend to recommend replacing them every 50K or 60K miles as preventative maintenance.

It may be time to replace your O2 sensors. I believe they should be replaced at around the 100K mile mark. I haven't replaced mine and I have 127K miles. From what I understand it is better to replace the pre-cat ones because they take the most abuse. The post-cat O2 sensors are more for reading and don't take the same amount of punishment as the pre-cat O2 sensors. Again, I'd replace the thermostat first and then see what happens.

With 120K miles I believe it's time for an Inspection II. Replacing the fuel filter is one of the tasks. It probably isn't related to your current problems. However, it should be done for preventative maintenance. Good luck.

Last edited by Chris C; 11-12-2007 at 09:56 AM.
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Old 11-12-2007, 09:19 PM   #3
djmcmath
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Thanks for the tips. To make sure I've got it right, here's the updated shopping list:

Water pump (and gasket)
Thermostat (even though it seems to be working correctly?)
Oxygen sensor (several?)
Fuel filter

Does that sound about right?

Dan
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Old 11-12-2007, 10:00 PM   #4
Akbrazil
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Hi Dan, I had the same issue and I went ahead and replaced the T-Stat. After the repairs, codes never came back.

As for your P0174 Code, I would go ahead and clean out your MAF. Pick up a can of MAF Cleaner (most autoparts store carry them)

Your part list sounds about right. You should also include a expansion tank (Its the tank where you fill your cooling system - located right next to the radiator) If you haven't notice, most e46fanatic member have had lot of issues with cracks and sometimes they just blow up. There a common replacement item you should include with your cooling overhaul.
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Old 11-13-2007, 01:18 AM   #5
Chris C
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I agree with 'Akbrazil' concerning the MAF Cleaner and the expansion tank. When I did my cooling system overhaul I didn't replace the expansion tank because I didn't know it was a problem area (do a search.) I plan on replacing it the next time I do a coolant flush. Anyway, I'd get the following parts:

Thermostat (correct P1620 & P0128)
Waterpump (preventative maintenance)
Expansion Tank (preventative maintenance)

Replacing the thermostat may take care of all of your problems. I would do that first along with the waterpump and expansion tank before buying O2 sensors. There are 4 O2 sensors in total (2 pre-cat and 2 post-cat.) If you need to, only get the 2 pre-cat O2 sensors. Again, I would only replace them if I had to. Good luck.
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Old 11-13-2007, 10:21 PM   #6
djmcmath
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I will probably replace the thermostat ... but it doesn't seem right to me. The Bentley's manual describes a couple of other codes that the system should throw if the thermostat fails. Additionally, I get varying resistance across the t-stat leads at different temperatures, indicative of an operational thermostat. That makes me suspect something else, but I don't know quite enough to know what else to suspect. ECT sensor, maybe?

Expansion tank seems like a strange thing to explode. Engineers brilliant enough to design a working VANOS engine should be able to make a coolant expansion tank that doesn't spontaneously disassemble itself. While I love driving this car, and I have thus far enjoyed working on it, it's these little oddities that make me wonder if maybe I shouldn't be driving an old VW instead.

What happens when the water pump gets old? What indications would I have of failure before it goes? How do I know that the PO didn't change out the water pump 15Kmi ago? Can I pull it, inspect it, and re-install it? I've read the procedure in the manual, but how long does it actually take?

Ok, so my budget doesn't support replacing everything all at once (which I would love to do), so I'm going with a more cautious approach:
1 - Clean the MAF, just because they get dirty and sometimes cause problems like that.
2 - Replace the coolant reservoir.
3 - Pull the water pump, inspect. Replace if necessary, re-install if not.
4 - Replace the thermostat.
5 - While I'm at it, I suppose I'll need to drain and refill the coolant system. Recommendations on fill fluid?
6 - Be prepared to spring for a pair of O2 sensors (I'm assuming that O2 sensors should be replaced in pairs, because if one is failing, the other is close?)
7 - "While I'm in there" I'm thinking I'll do the spark plugs and fuel filter.

Thanks for all the help everyone. I'm loving working on this car.

Cheers,

Dan
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Old 11-14-2007, 01:04 PM   #7
Chris C
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Have you done a carfax for your car? You may be able to get the information of the dealer that performed service on the car. That's how I was able to get my car's service history. I was lucky because the dealer wasn't terribly far away. I spoke with one of the service managers and he said that I could come in to get a print out of the information. I learned a lot about the car's prior maintenance which gave me peace of mind.

Waterpumps are generally replaced as preventative maintenance every 50K or 60K. You don't want a waterpump to fail on you in BMW. If you are at highway speeds and don't pull over fast enough to allow the engine to cool, you are looking at a costly repair (e.g. head gasket or maybe a warped head.) There is the Stewart aftermarket waterpump that is supposed to be pretty much bullet proof and really shouldn't need to be replaced at the 50K or 60K interval. However, I believe it cost ~$200.00 while a stock waterpump costs $60 or so. Perhaps some other folks can chime in on the Stewart waterpump...

The waterpump should begin to leak fluid before it fails or make a grinding/whining noise. When I replaced my waterpump at 116K the weep hole was crusted up and it didn't spin that freely. IMHO, I was lucky that it didn't fail on me. I replaced it just in time. I don't know about reinstalling the same waterpump. You probably need a new gasket/o-ring. Once you remove all the "stuff" to get access to the thermostat you only have a couple of more minutes to go to get access to the waterpump (i.e. remove belts and the pulley covering the waterpump.)

I'd replace the waterpump before the O2 sensors. And you are thinking correctly, I'd replace the O2 sensors in pairs.

Use either BMW coolant or Zerex G48 European formula (BMW approved coolant.) Don't use Prestone. There's the issue of phosphates and stuff. You can do a search for more info.

If you change the spark plugs, just get the OEM plugs listed for your car. I got mine from Advanced Auto and they were $6 or $7 per plug.

Unfortunately, BMWs do require a good bit of routine and preventative maintenance. If the car hasn't had it's Inspection I & IIs done, then the car will generally have drivability issues and may have costly problems down the road.

I wish you luck.
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Old 12-02-2007, 09:30 AM   #8
djmcmath
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Thanks for all the help! Here's the update: Replaced t-stat, waterpump, expansion tank, and fuel filter, and completed Inspection II. The t-stat related codes are gone. I'm going to run it for a little while and see if the "too lean" codes come back. If so, I'll have to replace an O2 sensor as well, I suppose.

The expansion tank was probably the original tank; it had the date of install on the side. I plan on cutting it open to inspect for damage. If I find anything interesting, I'll post it in one of the "I took apart my expansion tank" threads.

The water pump was also probably close to end-of-life, so it's good that I replaced it. The new pump has a metal impellor, but it was only about a $60 part, so I suspect it isn't the bullet-proof part.

Thanks again,

Dan
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