81 sc runs great for about 15 mins then....

[speedy]

I have an 81 sc abd it runs great for about 15 min and then rough running loss of power and stumbling. if I pull over let it idle for a min then power returns for a couple of min. also when I take my foot off the gas it feels like I am dragging a hummer.

my fv seems to be working and when I dicsonect the o2 sensor nothing happens
if I unplug my o2 sensor relay and then plug it back in I hear a click click and power is back for a couple of mins again. I also noted that if I drive it hard problem happens sooner
than the normal 15min.

I was thinking fuel....so I changed the fuel filter no change. so I thought I would hook up the
fuel pressure tester and drive around and run tests

1. my system pressure runs a little low at 4.1- 4.2 but should be at 4.5 -5.2
I took apart wur and backed out knock down pin and no real changes

should I be looking at the fuel pump??? how does one increase fp?? I took out the pin on side of fuel distr but not sure what do do with it.

thanks to anyone taking the time to read

[rsscotty]

You should do a flow check on your pump after the fuel filter in the engine bay. There are specs out there for volume of flow for your pump.

Also, I have seen problems with the filter in the tanks. These rarely get checked. Read the info in the below link.

Bosch K Jetronic

Hope this helps.

[speedy]

thanks.... I did see mention of it in the bently manual but thats all just a notes about it
thats funny I have had several 911s and never knew about. makes total sense. I sure hope its my problem. I will do both next weekend and report back
thanks again!

[Scott R]

That relay in particular gets very hot and will start to fail intermittently , not saying that you shouldn't look at the pump, but it's something else to check.

[R_Builder]

Fuel pump delivery spec for an '81SC is "at least 1000 cc (1 liter) per 30 seconds" per the tech spec book...

[speedy]

ok so I checked the pressure line off fuel acc and over 30 sec I got 1250 ish ml and min was 1000 so pump is working good. my local stealership can't get me a tank filter for about a week. my pressure gauge tells me i am at 4.2 bar so I though it might be a return issue? I added a washer at the fuel dist and increased my pressure to 5.2 bar I am going to get a thinner washer to bring in down a little..... now for the question of the hour
why didn't my mechanic do this 5 sec mod????? will it cause damage??

thanks

[UrQuattro]

Replace the o2 relay under the passener seat. If you unplug it and nothing happens, then that is a sign that the relay isn't functioning correctly. What should happen if you unplug it when its runnin is that it will display symptoms like you are describing, screw with the idle, cause small Backfires when decelerating, lots of weird stuff. I've been through exactly whatyou are talking about. I actually became convinced that it was the computer not the relay because i got a "replacement" relay at my FLAPS and the car worked a bit at first, and then it would start to run like crap.

But I highly highly recommend that you replace the relay before going any deeper into the fuel system. Worst case scenario, you're put the 17 bucks it costs for the relay, and you know that your relay is good for sure.

Also make sure to check that your interior light works. It could just be a fuse. But my bet is the relay.

Michael

[Bob Kontak]

A fellow last year had the same sort of problem. HE did all kinds of trouble shooting with the relay, etc and found that the connector on the back of the engine (firewall) had a poor connection that would break the circuit to the fv when the car warmed up.

Here is the thread - Post #46 for resolution.

911 SC running "flat" and no idle

[speedy]

so I took a drive to auto parts store to get a bolt to remove plug on gas tank and she died right in the parking lot. I heard kind of a pop sound from the front of the car. then no sounds of fuel pump working. she turned over and over. after ten mins she started up i am not sure how because the fuel pump was not running. same problems on the way home no power and sputter sputter from engine. I did make it back into the garage where
I started to screw around with fuel pump relay it would work off and on as I pulled relay in and out......so \i \took apart the relay and tightened up all connections still not working
there is power but not switching to power fpump. I gave up on that for today instead I decided to rule out the filter in the gas tank. It was clean. no problems there. once I get fp relay sorted I am going to dig into the lambda and fv. If I only had a match right now, I could solve my problems in seconds!

[mike635]

Sorry if this is obvious - did you check the fuse for the fuel pump? It's a 25A and naturally carries a lot of current. I had a look at mine the other day and it was quite corroded (but functioning) so I replaced it. Might be worth a look?

[speedy]

yes power is going through fuse the power is at the relay but its not switching over to turn fuel pump on. I tried different relay with no luck.

[speedy]

ok still runs like crap....same situation runs great for 10 -15 mins and then rough running and no acceleration!

new fuel pump delivers over 1250 ml of fuel over 30 sec
new fr relay
redid wiring around fp relay
fuse is good and dome lights work
new oxs relay
changed wiring at connection near firewall
new fuel filter
checked fuel filter in tank=good
took apart wur
all fuel pressures are on the button
checked pop of valve fo correct seating
air box seems good
tested cdi seems good

a couple of things I did notice is when I remove the oil cap the engine revs do not really drop. whats causes that? and when I test drive if I am experiencing the problem if I pull the o2 relay the problem goes away for a minute and then comes back.

any ideas suggestions???