Cold Engine Hesitation

LeppyGT · Post by **LeppyGT** » Fri Jul 11, 2008 12:54 am

When I first bought my 87 Gt, it got pretty nasty gas mileage. I replaced the fuel pressure regulator because a forum member from Old Europe said it was the

culprit. I then thought about it afterwards, and realized that obviously isn't it. I also unplugged the CSI, and kept it unplugged

until about a month ago. I did all this in Feb/Mar. So, when the car was cold, it was a real pain in the ass to drive. When it got warmer, the car

was still acting funny, and worsened, hesitating horribly all the time, warm or cold. I finally got tired of it and learned that the CSI only turns

on when the starter is engaged so I plugged it back in, and no difference. Turns out I ripped the FPR when I put it in, so I bought a new

one and put it in. Now its fine when its warm, but is still a MAJOR pain when the engine is cold. As I drive, it'll start to bog down, and if I keep

my foot on the gas, the car seems to cut out, and slow down hard, then out of nowhere, BAM! it comes back and throws you in the seat. Whats

the deal? Did something freak when I unplugged the CSI?

Aaron · Post by **Aaron** » Fri Jul 11, 2008 1:21 am

Nothing freaked when you unplugged the CSI. In fact, unplugging the CSI told you absolutely nothing, as if it were leaking, it'd leak whether it was plugged in or not.

LeppyGT · Post by **LeppyGT** » Sat Jul 12, 2008 12:16 am

Aaron wrote:Nothing freaked when you unplugged the CSI. In fact, unplugging the CSI told you absolutely nothing, as if it were leaking, it'd leak whether it was plugged in or not.

Yeah I know. I'm kind of new to cars, and so I didn't understand how the CSI worked. I've discussed this topic with you before. That's why I plugged it back in, because I now know that its useless no matter what. Just makes cold starts more of a bitch. What I'm wondering is why it still acts as if I haven't plugged it back in. Maybe I didn't get it on all the way, or something broke?

Mach10 · Post by **Mach10** » Sat Jul 19, 2008 3:45 pm

Honestly, just the usual culprits:

Check your timing, check your plug wires, distributor cap + rotor, and inspect all your spark plugs. Look for any that are darker/lighter than the others.

Is it harder to start when warm (cranks for a while or sputters?) It should snap on pretty quick.

Also, check the vacuum lines going to the MAP sensor for leaks. Disconnect the MAP sensor (this WILL throw the SES light) and take it for a spin in limp-home. Any improvement?

Finally, check your CTS: Measure the resistance across both poles when the car is cold. If Ambient is 10ÂºC you should be at around 4000ohms. If ambient is around 20ÂºC it should be between 2200 and 2800ohms. When the car is up to temp, disconnect it and check; it should be between 270-380ohms. A Bad CTS causes all kinds of weird behavior, and will murder your fuel economy.

LeppyGT · Post by **LeppyGT** » Sun Aug 24, 2008 6:41 pm

Mach10 wrote:Honestly, just the usual culprits:

Check your timing, check your plug wires, distributor cap + rotor, and inspect all your spark plugs. Look for any that are darker/lighter than the others.

Is it harder to start when warm (cranks for a while or sputters?) It should snap on pretty quick.

Also, check the vacuum lines going to the MAP sensor for leaks. Disconnect the MAP sensor (this WILL throw the SES light) and take it for a spin in limp-home. Any improvement?

Finally, check your CTS: Measure the resistance across both poles when the car is cold. If Ambient is 10ÂºC you should be at around 4000ohms. If ambient is around 20ÂºC it should be between 2200 and 2800ohms. When the car is up to temp, disconnect it and check; it should be between 270-380ohms. A Bad CTS causes all kinds of weird behavior, and will murder your fuel economy.

Okay, I tried taking the vacuum line off the MAP sensor, and the car basically just sputtered and died. I didn't even have a chance to get back in it. What exactly does the MAP sensor do?

And to further show my lack of knowledge, where is the coolant sensor? Is that the one in front of the valve cover, or by the CSI?

I've checked all the spark plugs, all that jazz, and no, when its warm it starts right up.

Xanth · Post by **Xanth** » Sun Aug 24, 2008 6:44 pm

Unplug the electrical connector from the MAP sensor, not the vacuum line.

lucky · Post by **lucky** » Sun Aug 24, 2008 10:51 pm

If you're standing behind the car, looking at the engine, the CTS is in the rear cylinder head, on your left.

Mach10 · Post by **Mach10** » Mon Aug 25, 2008 11:09 am

Pull it out, let it get to "room temperature" and check it against a thermostat or reasonably accurate thermometer...

Check against this table here:

You should be within 5% or so.

The car SHOULD be able to run without the MAP, albeight badly. Stalling out right away suggests that the sensor is at least marginally operational, and that the engine is already desparately trying to compensate for something. Try starting the engine with the MAP unplugged; if it idles at all, and shows any improvement, I'd say the sensor was feeding trash to the ECM.

I'd also check your TPS; make sure the resistance across the outside 2 pins (iirc) is smooth and consistant across it's full range (changes smoothly, returns to the same values at closed, open, and part throttle).

Aaron · Post by **Aaron** » Mon Aug 25, 2008 11:48 am

The engine will actually run without any one sensor.

I know the 3.4 DOHC will run without any one sensor, except for the crankshaft position sensor. When I unplug the MAP on a 3.4, the car idles fine and runs fine, until about 3500rpm, then it stumbles and won't climb any higher.

Mach10 · Post by **Mach10** » Mon Aug 25, 2008 12:01 pm

Aaron: It's DESIGNED to run with any one sensor out--assuming the rest of the motor and sensors are to blueprint spec.

Don't know if you've noticed, but the real world isn't quite that warm and fuzzy. What YOUR car does when you unplug a sensor is neither relevant or useful to this guy.