symptoms of a bad fuel pump relay

Kurt B · 12-20-2004, 07:52 AM

?
Car cranks and cranks but won't start now (84 Carrera).
For a few days, it randomly stalled when I shift from 1st to 2nd. Then it stalled and wouldn't restart for like 10 minutes. Finally got it started and back home.
Now, after sitting for a week, it won't start at all. I finally gave up.

Cranks and cranks, but there is absolutely no smell of gas anywhere. This car always has a little gas smell in the engine bay under normal conditions.

I know the DME unit, if you don't get the chip soldered in right, will act the same way, and I can check the solders, but since this kind of gradually went out, I'm thinking maybe the fuel pump relay went out.

I can disconnect the starter and crank the car and listen for the fuel pump, I know that, but I was wondering, for those who had a bad fuel pump relay, how it went out?

And should I smell gas when I crank and crank, but no start?

kach22i · kach22i's Garage

I've been collecting these, hope they help you.

http://forums.pelicanparts.com/porsche-911-technical-forum/196688-fuel-pump-911sc-aftermarket-replacements.html#post1658492

Pending fuel pump doom...

Rick Lee · Rick Lee's Garage

You mean the DME relay? That holds the fuel pump relay in a Carrera. Just swap it out. You need to carry a spare anyway. If that doesn't do it, it's probably the fuel pump, but you need to test it first by straightlining it at the fuse panel. I forget which pins, but use an always-hot circuit like the hazard lights and run a wire from it to the pin for the fuel pump. When I diagnosed mine, the pump coughed and barely stumbled.

kach22i · kach22i's Garage

I think there has been a rash of fuel pump/fuel pump relay questions lately. Could the colder weather be responsible?

Rick Lee · Rick Lee's Garage

No. It was not cold here yesterday and that's when I started worrying.

Doug Steinel · Doug Steinel's Garage

Rick Lee,

It wasn't cold yesterday in Fairfax, VA. How about today?

Rick Lee · Rick Lee's Garage

I haven't started my SC today, but it's pretty nipply here today. Still, my fuel pump noise has been very constant in all weather.

Kurt B · 12-23-2004, 05:32 PM

Well, here's an update. Today I started running some tests. I disconnected the starter and cranked it, and the fuel pump ran. I checked voltage to the coil, and it was charged. Did some of the tests for the DME harness, and they checked out--so the power is getting to the harness.

Then I pulled out the no 4 plug and setup a mirror so I could see if it was sparking when i cranked the car. Then it kinda almost wanted to start (with the plug out of all things, which was stupid)

I put the plug back in, and it did start. I drove it around the block. It wasn't running quite right, but pretty good considering it has sat for a week. I could kind of tell when i gave it gas it wanted to stall a little, but after running for a few, that seemed to subside.

Then I brought it back and pulled it up into the driveway and let it idle. Driveway is a little uphill.

Once I hit the gas a little, it stalled again and wouldn't restart, so I pushed it into the garage. It totally goes dead when it goes dead, usually by giving it more gas!

I guess tomorrow I'll check the fuel system again. i wonder if I got bad gas? Or is this going to be some weird problem that's going to take forever to fix.

autobonrun · autobonrun's Garage

I don't have a Bentley of the Carrera but some items come to mind that can be eliminated to help narrow down the problem.

1) I would just replace the fuel pump fuse in case there's a hairline crack in the filament that expands and opens up when it gets warm. At the same time, make sure the connectors holding the fuse are tight and have no corrosion.

2) A quick test for the relay itself is to swap it with say your power window relay. I think EarlySMan said that there are some minor internal differences with the fuel pump relay and the standard relay but it will work for your test.

3) You can also test the fuel pump relay two ways. 1) Have someone turn the key but don't start the car while your hand is on the relay. You can feel and hear the coil pick up. 2) Another way to test the relay is on the bench if you have two 6 volt batteries that are wired in series. I bought a couple batteries at radio shack that I keep for testing 12v circuits. Buy several jumpers with aligator clips at the same time. Pin 86 is the hot side of the coil and pin 85 is the ground. Apply voltage to these points on the relay and you should then see hear the relay pick up and contacts change state between pins 87A and 30. Pin 30 supplies the power to the fuel pump.

On my SC, I see the ground circuit for the fuel pump relay is associated with the alarm system, airflow sensor contact, and rpm limiter switch. If it is the same on your Carrera, it is feasible one of these items is interrupting the ground path causing the fuel pump relay to drop out. I don't quite have a good understanding of this part of the circuit but if the other tests are good, I might look at one of these items.

edit: I'm going to take a SWAG here. Hopefully someone with intimate knowledge of this portion of the circuit will chime in. I imagine that Porsche does not want to pump fuel unless there is also air being injected. If your air flow sensor is bad or going bad, it would interrupt the groung path shutting off the fuel pump. Again, it appears the alarm system and rpm limiter will do the same thing, but my first check of the ground side of the circuit would be with the air flow sensor contact. I know on my bimmer when I lost my mass air flow sensor, the car just shut down and would not start.

I'm not wishing bad luck on you but hopefully, you can do these tests at a point when the car will not start. Trying to find an intermittent problem is a real bear.

Good luck.

Kurt B · 12-23-2004, 06:32 PM

Thanks. I absolutely do need any ideas and avenues of attack for this, and those do help.
My only question is, is it randomly faulty spark, or randomly faulty fuel--or, the worst case, a combination!
It wasn't particularly good news that, after farting around with it all morning, that it started without having really done anything.

One think I do now remember is, after it started, I wheeled it out of the garage and it died and would not restart. I knew at the time I had half the air flow meter on. Just kinda shoved into the boot and connected. Once I screwed it on tight it started..but it wasn't loose again when it died the last time...I need to go over the AFM connections again just to be sure, but I don't think that's the issue.

Porsche Doc · Porsche Doc's Garage

Put a new head temp sensor in it.

And call me in the morning.

Kurt B · 12-23-2004, 06:37 PM

The day before it really started cutting out randomly and hard starting but before it went into non-response mode, i got a really bad fuel smell in the cabin. In fact, I didn't even want to drive it the next day. The next day though, it was bucking, but no fuel smell. Could be connected.
I should also check the gas cap. You never know.

Kurt B · 12-23-2004, 06:38 PM

Need a special tool and a trained spider monkey for that one..jeez, I hope I find out it's something else first.

Porsche Doc · Porsche Doc's Garage

Trust Me!

autobonrun · autobonrun's Garage

Quote:

Originally posted by Kurt B
Thanks. I absolutely do need any ideas and avenues of attack for this, and those do help.
My only question is, is it randomly faulty spark, or randomly faulty fuel--or, the worst case, a combination!
It wasn't particularly good news that, after farting around with it all morning, that it started without having really done anything.

One think I do now remember is, after it started, I wheeled it out of the garage and it died and would not restart. I knew at the time I had half the air flow meter on. Just kinda shoved into the boot and connected. Once I screwed it on tight it started..but it wasn't loose again when it died the last time...I need to go over the AFM connections again just to be sure, but I don't think that's the issue.

When my mass air flow sensor went bad, it did not completely fail. The car would sometimes start then quit at a stop light. Other times, the car would not even start in the garage. I finally performed some tests on the air flow sensor and determined it was bad. I'm not familiar with the Carrera setup, but recheck the sensor and the contact that is in the fuel pump circuit. If you know where the contact is located, you could possibly even short it momentarily if you know for certain you have air flow. At least then you would know the source of the problem. In my SC manual, the wires to the airflow sensor contact are brn/black on one side and brown on the other. Hopefully the Carrera's are the same or if you have a Bentley, see if yours are the same as the SC's.

You absolutely have to narrow the problem between fuel and spark. When the car is not running, make sure that fuel pump relay is picked up. If it is a spark issue, this whole assumption is off track.

Kurt B · 12-23-2004, 06:56 PM

Ok! A head temp sensor is on the menu then.
I'll keep you guys posted, thanks for the help!
(No. 4 spark plug was white tipped BTW..runnin way lean).

Kurt B · 12-24-2004, 03:59 PM

Quick xmas eve update. No good news, but I realized the car consistently gets spark--but that spark is pathetic 90% of the time (except when it wants to start).
It's small, and yellow. Not blue.
The coil is ancient. I did some measurments on it, and it partially checks out. Since the fly wheel sensors check out, I'm going to put a new coil in it.
It makes sense, or I'm hopeful it makes sense, that the spark is so pathetic, hitting the gas floods it and it dies.
Also, it always ran great at high RPMS..maybe it could get more energy at high RPMS.
Anyway, that's it. No place had n 84 coil today, so it will wait til next week. If that doesn't work, then I'll move on. But this coil is, if nothing else, old and a complete mess.

Porsche Doc · Porsche Doc's Garage

After you have bought all the wrong parts and have wasted all this time.
REPLACE THE HEAD TEMP SENSOR!

Early_S_Man · 12-24-2004, 06:26 PM

Kurt,

IIRC, didn't you change that head temp sensor to the new 2-wire version a couple of years ago???

Kurt B · 12-24-2004, 08:48 PM

I'm not sure Warren. I think it was replaced, but I didn't do it. I think a mechanic did it. I definitely didn't do it. Maybe TRE, but I don't recall. I see a plastic housing over it that looks basically new (whereas the speed sensors have a black housing).

LOL. Ok. The head temp sensor will go in first and I'll try it. Then the coil if it doesn't fire up!