Rough running when hot?!?

mike_the_man · 04-26-2003, 02:18 PM

Hi all,

I took my 74 914 for my second drive today. It ran great while it was cool, but after about 20 minutes of driving it started to run rough. It wouldn't accelerate smoothly, and it didn't seem to have much power. I am running the original fuel injection, and I have replaced the cap, rotor, points, condensor, plugs, wires, air filter, fuel filter, and cylinder head temp sensor.

It seems strange to me that it only runs rough when warm. What changes when the car is warmed up? Anybody have any ideas? Any thoughts or suggestions would be greatly appreciated.

Thanks,

chrisreale · 04-26-2003, 02:40 PM

Do you have the correct CHT? That is usually the culprit when rough warm running occurs. Do yo have the copper washer between the head and the CHT?

nyum96 · 04-26-2003, 09:38 PM

I had the same problem this week. I replaced the entire fuel injection a couple of months ago and could not understand whyI was having these problems. I had the vavles adjusted and it went away. Usually, when it runs rough when warm it could either be a MPS or tight vavles. Check the spark plugs, adjust valves, and the compression. I've learned that it isn't always the FI giving the trouble but then again, I'm not a master mechanic, just a broke consumer.

chrisreale · 04-27-2003, 12:42 AM

To my knowledge, the only reason a MPS would cause rough running, is if the diaphram is cracked, causing a rich condition. Of course, the rich condition would plague the engine at all times, causing cold starts to be tough as well. Id take some readings from your CHT, and make sure it is working properly. Chreck this site out to help you.
http://members.rennlist.com/pbanders/

nyum96 · 04-27-2003, 08:10 AM

Your right chrisreale, When I changed my CHT, she purred like a kitten at all temps, then the MPS went out. I have a different story. The car was fine when cold but ran very rough when warm. Fixed that with a new MPS, yes, NEW and idled at 850RMPs at all temps until this week. It just turned out to be a tight valve. What I'm saying is that it could be a different problem like a tight valve than a FI problem. Not too many people talk about what tight valves can do with a cars idle. We hear about engine destruction but if its a little tight, you will have the same results like too much fuel at idle when warm causeing the car to stop or idle poorly. Mike_the_man, take chrisreale advise. But if all is normal, this is what I did.

. Check the MPS
. Check for rust in the fuel tank (it'll cause a blockage at the bottom of he tank after 20 minutes, been there, done that = poor idle=no fuel) OFTEN OVERLOOKED!!
. recheck fuel filter for rust
. Check PCV
. Check fuel pressure (drivers side)
. Check return fuel line

. Read that rennlist site. I found his checklist after I did all my troubleshooting by myself. Then again all my problems were a combination at the same time.

chrisreale · 04-27-2003, 12:07 PM

Good advise.

jdogg · 04-27-2003, 05:40 PM

Adjust the valves first. I was shocked at what a good valve adjustment will do to how well the motor rus!!!

calbrat · 04-29-2003, 06:16 PM

I had a similar problem... Turns out one of the spark plug wires was backing off the plug a bit as it warmed up. This caused the car to stutter a lot for obvious reasons. Changed the wire and it has been good since.