Erratic Idle/No Start when engine warm

[Gallowlight]

Hey All,

I'm new to this forum, been on DriveAccord for a bit, but need more specific help.
I have a 2000 V6 Coupe with 300k that's been sitting for a while while I revamped the fuel system (Brake/Fuel Lines, Tank and filler/fuel hoses ect many things).

I have this issue we've been diagnosing. Idle for this guy has always been outta whack (read Idle high at cold start then drop), but drivable and reliable. After driving again after sitting, I would get an intermittent P0505 for the Idle Air Control Valve. Moving the car while in storage would get an occasional P0335 for the Crankshaft Position Sensor. Code only came on rarely.

We replaced the IAC valve, cleaned out the throttle body and IACV passages. We still got a temporary P0505 after replacing it, so we burped the coolant system. Still idling erratically in Neutral/Park. It would stutter at around 1,500 and hang there, or it would rise, fall, rise, fall and it moved was +/- 500 rpm. Car would sometimes struggle to start and kept stalling out. No codes would be thrown initially. twice over the last week we got a Crankshaft Position Sensor code after hours of driving and diagnosing. We further cleaned the idle air screw itself (bit of carbon buildup), as the adjustments we made to it did nothing. I checked the EGR for carbon buildup and it was clear.

We replaced the Throttle Position Sensor. We set it to a positive value based on data on our SnapOn Ethos Scantool. If there are specific values to set it too, please tell me. The old one had a -.6% value. We assumed a positive value was needed if the negative value was causing the issue.

On cold starts, it runs beautifully. Idle is where it should be (about 800/900ish) is the best we got. Driving great, no issues. But if you park and do a thing and come back and the engine still has some heat, it will either give you a cranky start but start then stall in gear, OR not start and crank and struggle to start. I had to drive home doing engine stands (gas and brake at the same time) to keep the RPM up and from stalling out.

We're debating replacing the main relay first(based on inspection), then the Crankshaft sensor (for possible intermittent failure), and possibly reinstalling the old IACV to see if anything changes. Does anyone have any suggestions or can give me some voltage/draw readings so I can further test the CKP/IACV/Main Relay? Many many thanks

 

[Gallowlight]

So I dug out the main relay (PGM-Fi/Fuel Pump) from under the dash. It did not look horrible with the solder joints, but two pins were fused together with solder, and the copper braids on the side of the relays had no solder left and looked a smidge toasty. I resoldered them and the car started and ran fine for the 5 minutes I ran it. Immediately threw the crankshaft sensor code again, so I'm going to order one of those. I will install that along with the new P/S Pump.

 

[Gallowlight]

Crankshaft Sensor fixed the fail to start/stall while idling issue. Starts and runs like a champ until its warm. P/S pump was installed, doing good there. Found out the lower radiator hose clamp was rusted and loosening, causing the car to lose coolant while hot. A new clamp solved that problem. Found the throttle and cruise control cables were too tight, which made the car think it was always under some throttle. We slackened those and it did improve. We found the IACV connector was wonky, the middle pin (signal) was loose, so we repaired the connector with new wires and solder. All of these small fixes made small improvements over the last few weeks working on it.

Last night after installing the repaired connector, we let the car warm up. While cold it wants to stay around 900 for the idle, but is smooth. But once warmed up, its hanging around 1,200rmp. After reaching operating temperature and the fans kick, the car will set a temporary P0505 code. Pretty reliably we can whack the throttle and get a permanent P0505. We notice a slight misfire, very slight, heard through the exhaust (Magnaflow). But it was running ok so we took it on a 2.5 mile test drive around the neighborhood and it ran great.

However we parked to see what might happen and playing with the wipers (raining slightly) and the power seat, I would see the instrument cluster lights flicker. But while sitting and giving the car gas it began idle hunting again. It would raise to 1,500 and drop to 1,000. Whacking the throttle it might stay at 1,200. But we noticed the temp gauge over heating, almost 3/4 of the way up the dial, but it was dancing there, moving a little up and down. No obvious signs of overheating, I have to check the level today to confirm. But the overflow tank was fine and there were no leaks that could be seen. Heat was blasting because it was cold and that did nothing to help the temp. I drove the short distance home and the temp came down very quickly driving. it was normal again when I got home. Parked and turned off and the fans stayed on for about 5 minutes.

So at this point were thinking a few things. One is that the IACV we got from Advance is not sealing properly, or is faulty out of the box. Two is that the alternator is overcharging and taking out sensors. Three is that there is still an issue with coolant. Either the thermostat/temperature sensor is failing or I have a headgasket leak. But nothing obvious yet. We keep coming back to the IACV and what would make it misbehave. Coolant seems the logical choice.

I am going to order a backup IACV from a better source, and possibly a block tester and alternator.

Any ideas please comment.

 

[Gallowlight]

Update. We installed another IACV, and the backup connector arrived. We soldered in the new connector as the one we fixed still had the power pin moving around interrupting the signal. The other IACV seemed to have some coolant passing by, not much but enough. We really cleaned the mating surfaces and reinstalled the throttle body. We found the PCV valve was plugged with oil (most likely original) and got a new one. Idle did improve, but when warm will keep around 1200 after revving. It did not overheat this time but did run warmer (halfway on temp gauge). I still believe an idle relearn needs to be done, but also that the car needs to drive and bleed out the air pockets. The heat is toasty warm on full blast, but possibly some sediment buildup in the system from over the years and needs to be flushed. I've owned it 9 years now and that's never been done in my owner ship. Gonna go drive and see what happens.

 

[Gallowlight]

Update. Have been driving the car occasionally the last month, putting over 500 miles on it. It has not overheated at all. Definitely needs a heater core flush, as on cold days you barely get cab heat. Blower motor was replaced (mice) and tons of new coolant is in the system. Idle continues to be a smidge erratic, and a dealer relearn is definitely needed. No codes get thrown and otherwise runs like a dream for the age/miles/crazy life. I'll update one last time once the relearn is done for final thoughts. Car probably just had to drive and work out it's kinks.