MFI - Rough when warm, great when cold

M491Cabriolet · 11:59 AM

My car is a 1972 911T. Does anyone know what would cause MFI to become rough, stutter, and backfire loudly whan it warms up? I have the hoses hooked up from the heat exchanger to the thermostat, as well as the one from the fan housing to force air through the heat exchanger. The car runs great, idles smoothly, etc.until it warms up. Then it becomes undriveable, really. The neighbors all throw themselves to the ground, thinking there is a sniper shooting at them. I mean, the backfire is so loud I think I lost some hearing yesterday! Any advice would be highly appreciated. I have the CMA manual. Thanks.

Danny_Ocean · 08-21-2008, 11:59 AM

The thermostat may be hooked up, but is it working?

M491Cabriolet · 08-21-2008, 12:04 PM

Good question. I have no idea, and I don't know how to check... How do you check this? Thanks!!!

FenderBender · FenderBender's Garage

Look here
http://forums.pelicanparts.com/porsche-911-technical-forum/269190-ultimate-mfi-resources-thread.html#post2441556

72 four door · 08-21-2008, 12:29 PM

They will tell you check, measure, adjust.

M491Cabriolet · 12:32 PM

That's what I figured. When you ask a banker for a loan, the safest and easiest thing for him to say is "no". When you ask supposed Porsche enthusiasts for help with an advice regarding MFI problems, the safest and easiest thing for them to say is "Read CMA".

I really don't want a doctorate in MFI. I just want help in getting my car to work properly. All I am trying to ask is...Is there any known setting or other problem that would cause MFI to spit and backfire loudly only when warm and run well when cold? Please help with advice. Please do not just tell me to do my own extensive research. I thought the benefit of a forum ike this was that the whole group is stronger than individual parts. (Together, we can help one another with our unique inputs, skills and talents). Thanks. Surely there is a good hearted Porsche enthusiast willing to help? Thanks.

kwikt 911 · kwikt 911's Garage

Did it always do this or did it just start?

Kevin

DW SD · 08-21-2008, 02:23 PM

Please give more background, if possible. Did this get progressively worse or happen after some incident (like a tune up)?

Spark issues can cause some of the same symptoms, as could a vacuum leak which worsens with temperature.

Diagnosing these things usually comes down to the basics. Are you sure you have good compression and spark?

Doug

M491Cabriolet · 08-21-2008, 02:42 PM

This is an engine that's really sort of unknown to me. It's a spare engine in the car while I'm rebuilding the original. It has been sitting for a long time and was not runing well at all. My mechanic started by putting in a 914 fuel pump, inline filters instead of the proper filter console, etc. I've been doing it step by step, and now the car feels and sounds great until it warms up. I will put in NGK BP7ES plugs,gap the points, time it, and measure and the balance the stacks, I hope this weekend. I think iI'll replace the vacuum hoses too just in case it might be a vacuum problem. I may take out the thermostat and clean it too. I was just wondering if this symptom I have is caused by a well known culprit. Thanks.

FenderBender · FenderBender's Garage

This is a good thread.
MFI, 1972 warm-up issues (combined)

I would check out the thermostat. If it is sticking it will stay rich after warm up. It is easy to remove and clean.

Zeke · 08-21-2008, 05:12 PM

Quote:

Originally Posted by M491Cabriolet

That's what I figured. When you ask a banker for a loan, the safest and easiest thing for him to say is "no". When you ask supposed Porsche enthusiasts for help with an advice regarding MFI problems, the safest and easiest thing for them to say is "Read CMA".

I really don't want a doctorate in MFI. I just want help in getting my car to work properly. All I am trying to ask is...Is there any known setting or other problem that would cause MFI to spit and backfire loudly only when warm and run well when cold? Please help with advice. Please do not just tell me to do my own extensive research. I thought the benefit of a forum ike this was that the whole group is stronger than individual parts. (Together, we can help one another with our unique inputs, skills and talents). Thanks. Surely there is a god hearted Porsche enthusiast willing to help? Thanks.

I think that's the best post I've EVER read here. I totally agree.

BTW, +2 on the t-stat. Best way to check that is to pull a spark plug after a warmed up run. Let it cool to cold before removing.

Danny_Ocean · 08-21-2008, 05:19 PM

Quote:

Originally Posted by M491Cabriolet

My mechanic started by putting in a 914 fuel pump, inline filters instead of the proper filter console, etc.

A 914 fuel pump is incorrect for MFI. Not enough pressure.

pwd72s · 08-21-2008, 05:45 PM

are your throttle bodies tight? Worn throttle bodies can cause rough running...

M491Cabriolet · 08-22-2008, 12:35 PM

I'll check the throttle bodies this weekend. Thanks. Danny, I should have been clearer. My mechanic was either inept or just cheating me. He charged $700 for the 914 pump and I had to replace it with a proper MFI pump, and I had to redo all the other useless work he did and charged me out the wazoo for too. Cheers.

fram914 · 08-22-2008, 02:19 PM

Try leaving the hose off the thermostat. If the problem goes away it must be running to lean when warm. There are external adj to compensate.

FenderBender · FenderBender's Garage

I am not recommending this, but what I did was removed the hose from the thermostat while car was running (warmed up) and moved the thermostat with a screw driver. You will tell right away if that is it, and how much it affects the mixture.

I agree with you on the CMA, but if you spend enough time working with it you will understand why the guys here preach it so much.

When I first started working on my car, I was telling some friends of mine that are diesel mechanics about it being such a PITA. One of them said, "I thought Porsche's were so precise". I paused and said "that is the whole problem it is too precise."

tobluforu · tobluforu's Garage

I vote/guess it's running lean?? When I had to get my 72 smogged in Ca I had to lean the crap out of it. It would idle awesome, run great when cold (about 5 minutes) but when warm and getting on the gas it would back fire/miss, etc, like crazy.

M491Cabriolet · 02:53 PM

It might seem ridiculous, but I changed the plugs to BP7ES, gapped the points and changed the timing, and the car no longer backfires at all anymore. It runs "well" whether cold or warm. Now the idle is high (1500 RPM). At least I was able to drive the car without scaring children and dogs... I think I'll bring it to a shop that can adjust the MFI to run properly now...unless someone can convince me that I can still do it on my own. I wish I knew how to reduce the idle speed, but it seems like a complicated process to get it right. Unlike CSI, it isn't one screw, right? I mean, each stack has to be balanced with the others. Is there an easyway to reduce the idle RPMs? Thanks.

FenderBender · FenderBender's Garage

No easy way that I know of. You can try the air correction screws. Also check the screw for the micro switch. Upper left side behind the flapper box. Some people try to use this screw as an idle screw and crank it in.

I hate to say it but it all comes back to CMA.

Sorry I just had to.