Perplexing No Go Problem

[Oredigger]

I am hoping someone can help me with a very mysterious problem on my 2001 XC70, 137k.

Usually after letting the car sit for awhile, when I turn it back on, the car will not go above 2500rpm/20mph a period of time. Then, all of sudden, it will work fine. The problem will come back, but only for a second or two during the same drive cycle. Aside from that, it seems to be totally random. (Sometimes you can turn it on after it has cooled off, and it will work fine, until it warms up, etc... )

Here is what I have done to the car recently:

- new plugs
- one new ignition coil (to replace one that failed)
- cleaned PCV system (required removal of the intake manifold)
- new Xemodex ETM
- new MAP sensor
- new MAF sensor
- new iPD HD TCV (turbo control valve) with new silicone vacuum hoses (just the ones to and from the valve)

Problems I still need to fix
- Brake booster vacuum leak
- Possibly need to clean intercooler from old plugged PCV system leaching oil into it
- Probably need to replace ALL vacuum lines with new rubber/silicone. I have found and put duct tape on several vacuum leaks.

I went to a local mechanic who had a Vida Dice, and he cleared the codes for me. Since then, I have had the problem numerous times, and no codes have been set. In fact, all systems check out fine according to the ECM.

I feel like it is an electrical problem, because when the problem stops, it is all of a sudden, like flipping a switch. But if it were electrical, I would think a code would be thrown.

I have not replaced any of the relays, but I have switched them around (ECM relay and Starter relay) and the bad behavior remains. So, either both relays are busted to the same degree, or the relays are not the problem.

[Oredigger]

This may be slightly related to engine temperature. It did it this morning, for quite awhile, until the entire engine bay came up to temp. The temperature gauge in the dash will jump up to operating temp pretty quickly, and the problem persists, but I think it may take some time for the heat to reach whatever part of the engine is causing this fault. This is just what I am thinking this morning.

Could this be related to gasoline octane? I have been running on on 85 octane (this is in Denver where 85 is the cheap stuff, I know they dont sell 85 at lower altitudes. So lowlanders, please do not freak out when you read 85. It does exist, and it is fine for us up in Denver). Anyway, I have been running it on 85 for quite awhile, with no problems, but that was with the old broke-arse ETM. I wonder if now that I am closer to achieving stage 0, the thing wont accept 85 octane. As I understand it though, if it were to detect a knock, it would just slightly retard timing --not something as drastic as reducing total engine rpms to 2500.\


Anyone have any opinions on my new theories?

[DGM]

It is hard to troubleshoot at a distance ...but have you verify if the temperature sensor is within range. Check the electrical contacts for corrosion. How old is the thermostat ?

It may be something to look at, just a few hints.

[Oredigger]

Hey thanks for the ideas DGM.

I have not checked the temperature sensor. Is this the one in the charge air intake pipe? Next to the MAP sensor? That one may be old and crappy. I have not replaced it yet.

I am assuming the thermostat is the original one, which would mean it was ten+ years old. Where are those located?

I swear, I think I may as well just replace every single sensor on the entire car before I am done. It seems to be the best way to troubleshoot.

Speaking of... I have a more general question, maybe I'll make another post. How much do you all think should be invested in these cars? I bought the Xemodex and the iPD HD TCV, because I have this wild idea that I can replace a few failing components with higher quality (more durable) components, and then the car will last for quite awhile. I am at 138k right now, but hoping to get 400k. Is this a good plan? Do you think it is worth investing in durable parts? Or are these cars intended to be "disposable" and recycled after 200k? Am I throwing my money into a pit? Should I just drive it into the ground because there are simply too many plastic/crappy electronics that will all eventually fail? Specifically, I am thinking about the ECU or ECM as it known. I can invest in a better ETM, all new sensors and whatnot, but once that ECM fails, as I understand it, I am SOL.

[DGM]

I was thinking of the water temperature sensor located beside the thermostat housing on the engine. Look at this link under "Engine Temperature Lower Than Normal + Cabin Heat Irregularities" to find general directions.

http://howardsvolvos.webs.com/engine.htm#650120623