3.2 Carrera will not start when hot (starts fine when cold)

911 in SC · 04-10-2011, 02:42 PM

I have an issue with my '84 and am trying to determine what to do next. I've searched the forum, but can't find what I'm looking for.

I've been driving my car 3-5 times a week, to work and back with no issues. I then drove it approximately 15 miles one day and went in for an appointment for approximately 20 minutes. When I went back out to go home, it wouldn't crank. After several tries, I put the pedal to the floor and it started with a lot of hesitation. It idled roughly, and ran roughly. However, I made it home with no problem.

I drove it again for the next week, to work and back, with no issues.

I then went for the same appointment after work. When I came back out, it wouldn't start. I tried several times, and it wouldn't start, although it turned over as usual. I pressed the gas pedal to the floor and tried to crank it and it wouldn't start. I tried it a few times and it still didn't start. I could smell fuel, so I figured I had flooded it, so I let it sit for 15 minutes or so.

I tried starting it again, and it wouldn't start. I pressed the pedal to the floor again and it finally started. It didn't want to idle, and ran roughly. I drove it home, and at times, it ran roughly, and then it would smooth out for a mile or two, and then run roughly again. I stopped to get fuel, and decided to leave it running, as I didn't want to be stranded at the pump. It idled fine for a minute or so, and then idled roughly. Just as I finished refueling, it cut off. I tried starting it several times and it wouldn't crank. I finally resorted to putting the pedal to the floor and tried starting it. After the second time doing this, it started, although it ran roughly and didn't want to idle. I drove it home, although it varied whether it ran good or bad. When running smoothly, it ran very good. Ran up to redline (or slightly before) very nicely. However, it wanted to cut off at part throttle. When I got home, I tried starting it, and it would not.

I decided to let it sit overnight.

I tried starting it the next day and it fired up as usual.

I believe it is either the fuel pump or fuel pressure regulator, but not certain.

Any ideas?

Thanks,
Mike

DRACO A5OG · DRACO A5OG's Garage

Swap the DME relay under the seat, if not then check the OHMs on the CHTS and both Speed & Ref Sensors.

911 in SC · 04-10-2011, 02:51 PM

Ahhh, I also swapped out the DME while in the parking lot. It didn't make a difference. Should I have left the new one in? I thought that changing it out would have made an immediate difference. Is that correct?

DRACO A5OG · DRACO A5OG's Garage

Quote:

Originally Posted by 911 in SC

Ahhh, I also swapped out the DME while in the parking lot. It didn't make a difference. Should I have left the new one in? I thought that changing it out would have made an immediate difference. Is that correct?

Yes, if a faulty relay, your symptoms would have disappeared.

Now you need to take out the DVM and check the OHMs of the CHTS, Speed & Ref sensor. Per Bentley specs

911 in SC · 04-10-2011, 03:01 PM

Will do. I'll check the Bentley for specs.

Is it possible the fuel pump could be going bad? I need to pick up a spare anyway...

DRACO A5OG · DRACO A5OG's Garage

I don't think so, you can make up a cheap fuel pressure gauge from home depot and pep boys and check the pressure.

It is said they only last 100K.

Why fix if it aint broken?

911 in SC · 04-10-2011, 03:25 PM

OOPS, I should have said I needed to pick up a spare fuel pump relay.

I certainly agree about not spending money when I don't need to. There's several things I want to spend the money on instead.

DRACO A5OG · DRACO A5OG's Garage

Check the OHMs on those components, a recent 2 members had similar issues with their Ref & Speed sensors after confirmation of failure. One replaced and is grinning again.

Mike80911 · Mike80911's Garage

I had the same problem with my 81 and as Draco said it turned out to be the Cylinder Head Temp Sensor

911 in SC · 04-10-2011, 04:38 PM

Thanks guys. I really appreciate it. Just have to find time to check them...

john walker's workshop · 04-10-2011, 05:34 PM

Quote:

Originally Posted by Mike80911

I had the same problem with my 81 and as Draco said it turned out to be the Cylinder Head Temp Sensor

81s don't have a head temp sensor.

yogi · 04-10-2011, 06:47 PM

I had a similar problem with my 87. It turned out to be the fuse to the fuel pump.

rusnak · 04-11-2011, 12:02 PM

Of course, no-crank is a separate problem with the starter or battery.

Rough running can be a symptom of a bad cht, but the best thing is to systematically check the fuel/ignition system, starting with the resistance checks on the sensors. Fuel pressure, and air fuel ratio should all be checked because it can be anything from a huge vacuum leak to a disconnected vacuum to the fuel pressure regulator, to a disconnected O2 sensor, etc etc.

Wil Ferch · 04-11-2011, 12:15 PM

John Walker...... I respect you, brother....but your statement about 81's not having a CHT sensor is correct.... but has nothing to do about the price of strawberries.

Original poster has an 84.... so it applies....

( just funnin' with ya ! )

911 in SC · 05-19-2011, 06:27 PM

Just a quick update. I was looking around the area where the connections are for the sensors. Right behind them, I noticed a small rubber hose with a 90° bernd that wasn't connected. I searched for the 'other end', found it, and connected it. I could be wrong, but I think it was the fuel pressure regulator. Anyway, I took it for a nice little drive, and then tried to crank it after it was nice and warm. It fired up without hesitation. I've driven it the past two days and it hasn't skipped a beat. I'll post again after I take it on the same errand that I went on when it first occurred. Thanks again everyone! I may not be out of the woods yet, but it certainly looks like it so far.

Mike