New Fuel Pump Issues

[jedoyle]

Hi Everyone

Looking to get input on a hot start issue I’m having. Drained the fuel tank and replaced the fuel pump on my 1983 911SC last weekend. Today I put a few gallons of gas in so that I could drive to the gas station and fill it up. Car was backfiring a bit on the way to the station due to the low fuel, but ran smoothly after filling up the tank. On the way home the car stalled while idling at a red light and won’t start anymore. The engine is turning over but I’m not getting combustion. Any thoughts on what the issue is?

[Otter74]

Have you checked the strainer in the tank? It’s possible you dredged up something from the bottom of the tank and the fuel supply is obstructed at either the strainer or the filter.

When I replaced my pump, I did not need to drain the tank - I clamped off the supply line and left a bowl under it to catch anything that made it through. This worked well and only maybe half a cup made it into the bowl.

[jedoyle]

Good point. I was thinking something might have gotten caught in the fuel injectors but I could first look at the strainer. There’s no way to check the strainer without emptying the 20 gallons of fuel I just put in, right?

[Jeff Alton]

Did you use a Bosch pump? If so it may have failed. That said, lots of great advice above that is all true and things to check. The strainer/filter in tank, and the tank itself can be an issue.

But, the Bosch fuel pumps from the last 5 years are garbage. Lost count of how many warranty ones we needed to replace.... 4 of the warranty replacement pumps were installs with a NEW tank installed as well.

Cheers

[Jeff Alton]

Did you use a Bosch pump? If so it may have failed. That said, lots of great advice above that is all true and things to check. The strainer/filter in tank, and the tank itself can be an issue.

But, the Bosch fuel pumps from the last 5 years are garbage. Lost count of how many warranty ones we needed to replace.... 4 of the warranty replacement pumps were installs with a NEW tank installed as well.

Cheers

[jedoyle]

Originally Posted by Jeff Alton

Did you use a Bosch pump? If so it may have failed. That said, lots of great advice above that is all true and things to check. The strainer/filter in tank, and the tank itself can be an issue.

But, the Bosch fuel pumps from the last 5 years are garbage. Lost count of how many warranty ones we needed to replace.... 4 of the warranty replacement pumps were installs with a NEW tank installed as well.

Cheers

You’re exactly right it’s a Bosch fuel pump. I can hear it beeping when I turn the ignition though, does that mean it’s still working? Maybe I should back to my original Bosch. It was working just fine I figured I’d just replaced it proactively.

[Jeff Alton]

Originally Posted by jedoyle

You’re exactly right it’s a Bosch fuel pump. I can hear it beeping when I turn the ignition though, does that mean it’s still working? Maybe I should back to my original Bosch. It was working just fine I figured I’d just replaced it proactively.

I would check the tank, make sure no debris is getting into the pump. Then make a choice on the pump.

Cheers

[A930Rocket]

What’s the alternative to a new Bosch fuel pump?

Would it make sense to put a fuel filter between the tank and fuel pump, to catch debris early?

[Otter74]

That’s what the strainer is for. And the pump is located immediately downstream of (and next to) the tank, so there isn’t really any room for anything else in-line.

[Jeff Alton]

The Bosch pumps do not fail from debris any more that any other pump would. They fail from being ****ty. SC/Carrera pump or even the 930 pump. The last one we just replaced a week ago was a 930 pump in a 3.2 Carrera we converted to a custom turbo set up. New fuel tank 18 months ago......


We are still searching for a better more reliable replacement. As this warranty stuff in annoying. Ya, we get a new pump for free, and we get paid a very small percentage of our labor to replace it. But its a waste of time for our clients and it still costs us money as the lifts and manpower could be used doing profitable jobs.

Same goes for any Bosch Reman part, be it a starter or an alternator. Stay away from it. As a DIY guy you are better off sourcing a local business that can rebuild it. Every city has multiple shops and every small town has at least one shop that can do the job.

Cheers

[porschedude996]

Originally Posted by jedoyle

Good point. I was thinking something might have gotten caught in the fuel injectors but I could first look at the strainer. There’s no way to check the strainer without emptying the 20 gallons of fuel I just put in, right?

I doubt that the fuel injectors have dirt from your recent events. There is a huge filter before the CIS fuel distributor. Somewhere between the outlet tube in the tank and the fuel filter is most likely if it is a fuel supply issue. If you have a fuel injection gage set, check the supply pressure and the flow volume. If wrong or no flow, check for output on the discharge side of the pump. If none or low, check the inlet side of the pump. You may be able to use a hose pinch pliers and pinch off the inlet hose on the pump and see if it is clear. Rather than draining the tank. Remove fuel level sensor and check for debris.

If you make a jumper with a switch, you can test pressure and flow with the engine static.

How to: https://forums.pelicanparts.com/porsche-911-technical-forum/1132263-fuel-pump-relay-bypass.html

[jedoyle]

Update for everyone. I pulled the fuel tank drain plug and that looked fine. I also tested fuel line going into the fuel accumulator and I was seeing fuel, so the fuel pump seems fine. Any other thoughts? Should I test the line going into the fuel distributor?

[E Sully]

You could check the fuel pump delivery rate at the return line to the tank. It should flow 1,000 cc in 30 seconds.

[Mark Salvetti]

Going back to the original post, I don't think low fuel should cause the car to backfire. It is something else. Are you sure you have spark?

Pull an injector (any injector), put it in a jar and with the pump energized, lift the metering plate in the air box. Is there fuel spraying out of the injector? Don't do this for long, if there is fuel the other injectors are dumping fuel into the cylinders.

After that, it's time to check if you have spark.

Mark

[jedoyle]

I can confirm I’ve got spark and the fuel injectors are spraying fuel. Am I correct in assuming it’s most likely a timing issue?

[PeteKz]

No. Incorrect timing would cause poor running, but not no-start.

Go back to basics: Compression, fuel, spark. Your compression is probably okay, but I would check it anyway to rule it out.

Then fuel: Jumper the pump so it runs when you turn on the ignition. Then remove the air filter and stick your finger up in the air metering unit and raise the arm. You should hear the injectors squeak. Only do this for a couple seconds, or you will flood the engine.

If you have compression and fuel, then it's loss of spark. Recheck everything.

Based on your description so far, I think you are losing spark. It may be a problem when the car warms up and the ignition coil or CDI box get warm. See if the car fires when it is cold.

It may be possible that you flooded the engine during your starting attempts, and that's why it won't start. If you haven't done the compression test, then remove all the spark plugs and crank the engine through about 10 cycles. That should clear the flooding too. If flooded, that may be due to a stuck cold start valve or something related.