Hello all,
In early December we had a real cold spell (almost thought I was in MN) and on the low of lows (-10 or so) the thermostat locked closed.  I figured out htat the heater wasn't working and made the guess about 15 miles later.  Anyhow, after replacing the thremostat, it started to run very poorly after it reached something in the range of warm.  What it does is start to miss badly, especially below 3000 rpm.  It will idle down for a few seconds, then die if I don't keep the rpm up (after ti gets warm).

I replaced the temperture sensor for the computer, as well as the MAF unit (found one off of an X90 reasonably priced).  I have an OBD2 code reader and had nothing to read for the first week or two except 'engine missing' -- duh!  The other day it had 4 failure codes, including the temp sensor, barometric pressure, MAF, and something else (don't have the list handy).  I pulled the plugs:  All are very clean-- no brown or black (just a couple of months old; strikes me as too clean).

I am tempted to at least get a large resistor to match the temp sensor and ground it (got to look up the circuit to make sure it goes to ground, etc).  At least it should run okay. 

What I really want to do is fix it.  Vehicle has 148000 on the clock and is a 5 speed.  Any ideas?

I'm thinking a blown head gasket, from the
coolant not cirulating from a blocked T-stat

head gasket might leak into the oil,he could look there, or it could be water in the fuel (cloging the filter) we have that problem here alot, I can hear my fuel pump grinding it up when cold out  (I know i'm too cheap to buy a bottle of heat) we have alot of bad gas stations here.

Will have to check compression.  Not using any water, but maybe just lucky.  Still seems strange to have such clean plugs.  May check fuel pressure anyhow.

Thanks for some starters.

Will have to check compression.

leakdown test would be a great idea also

Update:
Checked the valves, three exhaust .001 tight.
Torqued the head bolts I could get to (all but 1).  Found the exhaust side bolts a bit loose (~30# or so).
Compression check yielded 110-125#.  5 minute leakdown ~2-4#.  Book says 170-199 psi.  All measurements taken around low to mid 40's, engine cold.  Don't now the accuracy of my gauge (probably close), and at 6000', my levels would be a little low but I don't remember the correction multiplier.  Anyhow, no great changes in operation.  Also verified the cape and rotor have no cracks, or obvious distributor play.

Any thoughts out there?

The white plugs and the ruff idle and poor running below 3k allomst sonds like a vac. leak, could be a sticky egr valve?

Will look at that tomorrow -- thanks.  IIRC, EGR doesn't work much until the engine is warm?  Or am I thinking of it when it normally functions, as opposed to the head is warm and the thing is loose?

If the egr valve is bad it is like a leaky faucet, hot or cold water does not matter     ya the shaft on them gets alot of carbon build up or a piece broke off and lodged itself in the seat. just a guess.

Well, no joy on the EGR circuit.  Tested all components; when pulling vacuum on the egr itself, engine speed drops. 
Thinking that since it is sensitive to engine temp (depending on ambient temp, U get as much as 5 minutes before the idle starts to miss) I bypassed the intake air bypass (may be using the wrong term here:  it mounts at the front of the intake) coolant lines (they are supposed to heat up a wax pellet and cause the bypass to close ~ 158 degrees).  Didn't work any better (or worse .
Going to see if I can find a fuel pressure gauge and adapter.  For what it's worth:  if the engine is cold, I would bypass removing the connector on the fuel pump relay (it's WAY up in the dash & painful to access).  Just put a rag over the plug as you remove it (you should anyhow to catch the dribbles).
Getting flumoxed here.  Hate to have to give in and take it to someone.
Curious, anyone have any experience plus/minus on rebuilt engines an/or sources for them (long block)?

No joy on the fuel pressure.  Runs according to the book (high 30's to low 40's).  No new codes (just the 'multiple cylinders miss firing' 0300 code).
Put a DMM across the TPS and it all looks good.
Did notice that the 'fuel open' sensor reading changed to closed after playing with the EGR stuff (now toggles like the book says it should).
I changed the forward O2 sensor as  shot in the dark.  No change.
Wondering, wondering, wondering (similar to thinking, with the same results:  nothings happening).  Under 3000 rpm (and at idle) irt misses, badly.  At least now it mostly doesn't die at idle.  Just takes a while to 'spool up' when left at idle.
I sprayed starting fluid around the intake and got no idle speed changes.   Need some ideas before I have to actually see a mechanic.  Shame the thought!

Could be dirty fuel injectors? leaking on idle would run good while cold and ok while driving over 3k rpm's, some times when rubber fuel lines start coming appart the dirt or broken piece of hose will lodge in the injector if the screens on them are not 100%, can you borrow a hand held pyrometer(point and click type) and see how your exhaust manifold temps are?

Nada on the temp meter.
Will have to see if the book tells me how to remove/inspect the injectors.  I'm wondering if I can make a device to test the spray pattern, etc, at home ( I know there were several dealer only tester based tests
Still seems like I am missing a critical clue on the start of the problem, and the way it is working.  To recap, local overheating on an extremely cold 30 mile trip.  Now it misses when warm, but only after temp sensor is in the mid 130's and up (as I recall the code reader saying).

When my tps was acting up I would just unplug my temp sensor and run the ecm in open loop, it ran fine, not with all of the advance and fuel control but it had no miss at all.

Tried it.  No change.  Still need to pull an injector.  This thing reminds me of a motorcyle I had that had a bad coil.  It would run great at high rpm, not at low rpm.  But it had two coils.  May have to put the timing light in it and see if it misses along with the engine. I MIGHT see missing spark that way, and at least be able to rule out the fuel system.

Got to be missing something basic....