Do we have any GM Certified mechanics on here?

[Doward]

I've got a question, this has been pissing me off.

Long story short, I've got a Chevy Colorado, 3.5L I5, that I firmly believe falls under GM's valve seat problem. GM sent me a letter, stating I've got a 100k warranty with which to get the seats fixed, if they are a problem.

Symptoms:
1) Check engine light, throwing a P code (damn if I can remember the number - I'll actually go look it up again tonight) that comes up as "Idle too low"
2) Extremely rough idle, that has gotten worse and worse with age
3) I bought the truck @ 35k miles, now has 55k miles. The idle has gone steadily downhill.
4) I brought the truck to Palm Chevy in Gainesville. They tried to tell me it was a MAP sensor (I hooked HPTuners up to the truck, and the MAP reads perfectly fine) and would be $384. I told them no. (They also claimed a bad MAP code showed up, and neither my handheld OBDII reader nor HPTuners verified that)

Of course, I called GM up, and complained, trying to find out what I can do about this. They basically said they need a GM Certified Mechanic to agree, and then GM will pay for the repairs.

I'm trying to figure out what options I've got here. Am I out of line in demanding that the dealership show me, step by step, their diagnostic procedure for determining the 'map' is bad?

I also thought about replacing the MAP sensor (it's $84 from the parts department of the SAME DEALERSHIP) - then when they try to tell me it's bad, throw it back in their face that I just bought it from them.

I really fucking hate dealing with dealerships, and I refuse to let them charge me for $400 for a $90 part, that I know is good.

Options?

 

[jmanbball]

I'd have them compression test, then leak down test the engine, while your standing there.

 

[ilikebigbutts]

how do you know the map is good for sure?really the only way we make sure is by comparing barometric pressures on the scanner on suspected car and one that runs fine.i've seen some baro's that read close but still make the car run like shit.if the sensor is easy to replace you could ask them to prove it to you by quickly hanging one on or borowing one from another vehicle in the shop.would they still charge you if what they did doesnt fix your problem?

i work for dodge btw, keep flaming at a minimum.

edit:bad map doesnt have to throw a code.if the problem isnt wiring and the reading is not that far off, most likely it wont pop a code.

edit # 2: also i've seen map's bad that cause the car to run like shit, not just idle.

 

[ilikebigbutts]

this is out of the dodge caravan online manual, read bold

OPERATION
The MAP serves as a PCM input, using a silicon based sensing unit, to provide data on the manifold vacuum that draws the air/fuel mixture into the combustion chamber. The PCM requires this information to determine injector pulse width and spark advance. When MAP equals Barometric pressure, the pulse width will be at maximum.

Also like the cam and crank sensors, a 5 volt reference is supplied from the PCM and returns a voltage signal to the PCM that reflects manifold pressure. The zero pressure reading is 0.5 volt and full scale is 4.5 volt. For a pressure swing of 0 — 15 psi the voltage changes 4.0 volt. The sensor is supplied a regulated 4.8 to 5.1 volts to operate the sensor. Like the cam and crank sensors ground is provided through the sensor return circuit.

The MAP sensor input is the number one contributor to pulse width. The most important function of the MAP sensor is to determine barometric pressure. The PCM needs to know if the vehicle is at sea level or is it in Denver at 5000 feet above sea level, because the air density changes with altitude. It will also help to correct for varying weather conditions. If a hurricane was coming through the pressure would be very, very low or there could be a real fair weather, high pressure area. This is important because as air pressure changes the barometric pressure changes. Barometric pressure and altitude have a direct inverse correlation, as altitude goes up barometric goes down. The first thing that happens as the key is rolled on, before reaching the crank position, the PCM powers up, comes around and looks at the MAP voltage, and based upon the voltage it sees, it knows the current barometric pressure relative to altitude. Once the engine starts, the PCM looks at the voltage again, continuously every 12 milliseconds, and compares the current voltage to what it was at key on. The difference between current and what it was at key on is manifold vacuum.

During key On (engine not running) the sensor reads (updates) barometric pressure. A normal range can be obtained by monitoring known good sensor in your work area.

As the altitude increases the air becomes thinner (less oxygen). If a vehicle is started and driven to a very different altitude than where it was at key On the barometric pressure needs to be updated. Any time the PCM sees Wide Open throttle, based upon TPS angle and RPM it will update barometric pressure in the MAP memory cell. With periodic updates, the PCM can make its calculations more effectively.

The PCM uses the MAP sensor to aid in calculating the following:

Barometric pressure
Engine load
Manifold pressure
Injector pulse-width
Spark-advance programs
Shift-point strategies (F4AC1 transmissions only, via the PCI bus)
Idle speed
Decel fuel shutoff
The PCM recognizes a decrease in manifold pressure by monitoring a decrease in voltage from the reading stored in the barometric pressure memory cell. The MAP sensor is a linear sensor; as pressure changes, voltage changes proportionately. The range of voltage output from the sensor is usually between 4.6 volts at sea level to as low as 0.3 volts at 88 kPa (26 in. Hg). Barometric pressure is the pressure exerted by the atmosphere upon an object. At sea level on a standard day, no storm, barometric pressure is 101 kPa (29.92 in. Hg). For every 30.5 meters (100 feet) of altitude barometric pressure drops .33 kPa (0.10 in. Hg). If a storm goes through it can either add, high pressure, or decrease, low pressure, from what should be present for that altitude. You should make a habit of knowing what the average pressure and corresponding barometric pressure is for your area.

 

[cheverlot]

I am a gm and dodge certified tech. I had a on with the problem you need to have your dealer do a leakdown test per si. tell them that you have the campain 07123 and you want a cyclinder leakdown test performed.

 

[Doward]

10-4 Cheverlot, appreciate the information.

ilikebigbutts, I compared the output from 15 kPa to outside pressure (which was 101 kPa) and verified a good clean signal. You can see the MAP output in realtime on HPTuners

I'll go bang heads with them again here in the next few weeks.