Hello,

I've been searching all over the net and have found zero information on this one. I have found good suggestions on here which didn't lead to a fix but you guys seem to know quite a bit so I thought I would post here and ask for advice.

I have a 2004 Pontiac Montana here in Michigan.

2 summers ago (i.e. not this last one, the one before) we had a random overheat while driving on a 95+ degree day - according to the thermostat that is.

I immediately pulled over and opened the hood to find no boiling whatsoever in the coolant bottle, no antifreeze smell, nothing.

We waited a while and went on to our destination a few miles away. It was a graduation party so after 5+ hours we left and didn't have the issue for the 60 mile trip home.

I pulled the air filter the next day to find the box was 98%+ full of cotton on the intake side of the filter with a hole about 4 inches around for the air to flow into the intake. We bought the car from a repo place and since it was sitting it was packed full by mice apparently. Although we had it for about 6 months before that without issue. I only bring this up because it was like insulation on top of the ECU so I'm wondering if that may ultimately be the problem (over heated ECU) - more on why I'm thinking that in a minute.

I cleaned out the box, put in a new thermostat while I was at it since I figured it couldn't hurt because it was probably the original, and put in a new air filter.

Things were good for a few months and then the temperature gauge would randomly bounce up and down from completely off and sometimes up to fully over heating. When it went to off the RPM'* would go up, on the RPM'* would go down.

It continued this and this last summer we started getting the random over heating and actually getting boiling this time. We also would have random hard starting issues.

I changed out the temperature sender with a new BWD from Advance Auto and nothing changed. I then changed out the IAC. The old IAC was all blacked up with a gooey tar like substance and noticeably worn in the cone. I cleaned out the chamber for the IAC and put the new one in. Also changed the MAP sensor and the TPS. I figured that potentially they could be contributing to the hard start and they were cheap (and likely original - van has just over 180,000 on it). At this time I found the vacuum line from the fuel pressure regulator was broken in 4+ places from being heat soaked over the years and brittle. I replaced the vacuum line at this time.

One of those things kicked the fans on and that'* when it occurred to me that the fans had not been running on high when the van was getting hot. I also drained, flushed with water (no detergents) the radiator and refilled it with new fluid).

The overheating has stopped but the hard starts when hot got worse and worse to the point that it would not start at all if we drove it over 30 minutes and tried to restart. We had to let it sit for 30+ minutes before it would start again.

We had allot going on and we parked it for the last 3 - 4 months. A few weeks ago we decided to take a driving trip to Disney (Leaving tomorrow actually) so I started getting more aggressive with trying to fix it.

My wife had taken it to Autozone and they said the computer would not give them any information despite the SES light being on. So I figured something with the wiring (should have thought about this more instead of trusting the apparent idiot at Autozone - more on this in a minute).

So I read up and checked voltages on the MAP, MAF, TPS, and ECT Sensor. All fine. During this check I found the black vacuum hose that is about a 1/2" in diameter that goes from next to the IAT on the intake hose back to the back side of the engine was loose on the engine side. Upon inspection I found that the rubber plug on the back side was deteriorated really bad and not sealed. The hose was full of a light brown gooey / oily substance. I got a new condition used one and put it in place (making sure to install and turn so the key has it locked in place on the back of the engine). I also read that the ridiculously hard to reach Crank Sensor on the back of the engine block could be causing the issue due to it being old and potentially brittle. So I replaced that as well. *** Personal tip, remove the intake hose and MAF then reach around from the front to the back to change this - MUCH easier than trying to get it from underneath or removing all the exhaust shield like GM suggests. ***

Still hard start when hot.

I read one of the other posts on here and figured it must be the fuel pump pressure (didn't have a tester myself to check) because I didn't hear it run anymore at the Accessory on position in the ignition. So we finally got that replaced late last week and the fuel pump is definitely running now but still the hard start when hot issue.

I then smelled the vacuum line from the Pressure Regulator to the MAP and it smelt like gas. So I went up to Advance Auto and picked up a new one.

On my way there the temperature gauge was doing it'* ceremonial jumping around and then weirdly it just stopped and dropped to 100 degrees. No more bouncing at all.

While at Advance I asked them to scan the vehicle and they got nothing on the plug. The guy says "Oh, that means your fuse is blown" (hence the reference to the idiot at Autozone earlier). So I open the panel, yep blown fuse, replace it and get an error code for "High Input voltage at the ECT". That was the ONLY code in the system.

So bad ECT right from the start was my guess so I picked up another one and went home.

Installed the new fuel pressure regulator and pulled the old ECT. Checked it at ambient and got 5.9 ohms (probably on the wrong scaled on the multimeter so convert to whatever it should be). The new one read at 3.9 ohms. So I installed the new one.

Still have the same hard start at hot and the Temperature gauge still stays at 100.

I read another post somewhere that someone had the same issue and went up and had their codes reset. As soon as they did on their vehicle they indicated that their fans came on and their temperature gauge functioned. The shop that person was at indicated that the PCM must have seen the fluctuation and shut down the ECT to protect the engine.

So, I went back out and checked for voltage at the ECT and I have no voltage at the ECT, I do have 5 volts at the MAP and IAT. The cooling fans are on 100% of the time at cold or hot so I suspect it is something with the PCM protecting the engine because it'* not getting a reading from the ECT.

So we went up to the store and had the codes reset. Unfortunately though the temperature gauge did not come on and I still have no voltage at the ECT. Shortly after we left the store the SES came back on so we turned around and went back to the store to get the code. This time the only code is "P0114 - Intake Air Temperature Sensor 1 Circuit Intermittent Bank 1" and apparently there is "No possible causes available" and "No reported potential fixes available".

I rechecked the IAT and I still have 5 volts and no voltage at the ECT.

When you cycle the key the temperature gauge goes down below 100 then back up to 100 so it'* functioning in the instrument cluster.

I read somewhere that the TPS, ECT and IAT share the same "Common 5 volts" but I'm not sure if that means they have the exact same power source or not?

I have tried additionally checking for continuity on the connector and I have continuity on the one side but no continuity on the "Yellow" wire.

So I'm stumped at this point.

I'm not sure if there really is a shutdown type code inside the PCM that shut off the power to the ECT and that somehow needs to be reset by a method other than just clearing the codes?

Or perhaps the PCM is bad because of the original overheating problem with the cotton buildup.

Or perhaps because I have no continuity at the connector the wire is broken somewhere?

All of which doesn't really make sense to me with the only code now being related to the IAT - Unless the PCM is bad?

The van is definitely idling higher (about 1,000 rpms) and it seems to be getting really poor gas mileage.

The hard start seems to be because it is flooded. During the hot start, if we hold the pedal to the floor it will usually stumble, run rough and then clear itself out and run fine. I read somewhere that it'* likely the ECT issue because the PCM thinks the vehicle is cold and dumps extra fuel into the system to get it to start but because the cylinders are already hot the system doesn't need the extra gas and is effectively flooding it out. Hence why when the system sits for several hours and the block is actually close to ambient temperature the system fires up without issue.

So basically the questions are:

1.) Should the ECT yellow wire have continuity to the block?

2.) If so, would it have continuity even if the PCM had switched it off (I would think not but I don't know anymore)?

3.) Is there really a situation where the PCM would shut down the 5 volt signal and if so how do I reset it?

4.) Is there a way to check the PCM to see if it'* bad?

5.) Is there a way to check the IAT to see if it'* bad (i.e. resistance test for example)? ** I don't think this is causing the temperature sensor issue though

6.) Is the 5 volt line independently linked to the PCM for reading the signal from the ECT? I'm wondering which leg reads the signal from the ECT?

-- My thought process is that if the yellow is just a "Dumb" 5 volt line (i.e. on / off 5 volts and the other wire routes the signal to the PCM) then perhaps I could jump the 5 volts from the IAT to the yellow on the ECT connector and see if that gets the temperature sensor working. I don't want to try this and then for sure fry the PCM.

7.) I read something about connectors under the vehicle near the rear of the drivers seat that could cause hard starts but would that have anything to do with no power from the PCM to the ECT?

Thanks for any advice! I truly appreciate it since we are suppose to be leaving for the drive from Michigan to Florida tomorrow!

Sorry, one other question I missed above.

8.) If I replace the PCM with one from a junk yard do I only have to do the "Security Relearn" or do I have to take it to the dealer to have them do the "CASE" relearn (I don't know what the case relearn is to be honest)?

seems like the 5v reference is shorting out, i would be looking for wires with bare insulation in the engine harness. if you replace the pcm it needs a high end scanner or dealer tech 2. i would go through the wiring with a fine tooth comb first.

this diagram is better make sure the feeds and returns are good with no shorts or abrasions

Thanks JWFirebird, that'* what I was thinking but that one post I saw on the net was confusing me.

It looks like the 5v is common so as a "short term" fix I could jump the 5v from the IAT to get juice to the ECT right?

give that the returns are tied together i would assume the +5v side is the pcm monitored side

DTC P0117
Circuit Description
The engine coolant temperature (ECT) sensor is a variable resistor, sometimes called a thermistor, that measures the temperature of the engine coolant. The powertrain control module (PCM) supplies 5 volts to the ECT signal circuit and a ground for the ECT low reference circuit. When the ECT is cold, the sensor resistance is high. When the ECT increases, the sensor resistance lowers. With high sensor resistance, the PCM detects a high voltage on the ECT signal circuit. With lower sensor resistance, the PCM detects a lower voltage on the ECT signal circuit. If the PCM detects an excessively low ECT signal voltage, which is a high temperature indication, DTC P0117 sets.

Sorry for the long delay between posts. I was unemployed and right after our trip (actually got the call while on the trip) I got a new job and have been working.

We wound up having to rent a van for the trip because I ran out of time to fix ours but I got right on it when we got back and I'm happy to say that thanks to jwFirebird we have been up and running for about 6 weeks now with not a single issue!

Solution: I was able to use the wiring diagrams to determine that there was voltage leaving the PCM in the direction of the ECT sensor it was not making it through the harness to the plug. Because GM made the wonderfully stupid decision to run the harness between the block and the intake I could not get to the harness to find the open and repair it. So, I pulled the PCM, cut the wire for the ECT about 8 inches from the plug, cut the wire about 4 inches from the ECT sensor plug and soldered a splice wire between the two of them.

It'* been running fine ever since with accurate temperature readings and not a single hard start!

Thanks again jwFirebird for all your assistance and advice!

thanks for posting your fix

Great news, glad to hear you were able to get it fixed!