Fuel Pressure 1.6 to 2.2 bar -- help?

[piscator]

EDIT: 15 June 09 Warm Start Problem Solved! - with the great help of the 'Pelicans' posting here! This thread outlines the fuel pressure testing that uncovered a faulty check-valve and lead to replacing the fuel pump. Warm-start problem GONE!

I did a fuel pressure test today on my 1978 911SC Targa. I suspect a defective fuel pump and would really appreciate some help looking over the data.

Symptoms: My SC starts right up when cold, or immediately after shutting off, but won't start easily 1 hour after shutting off. I take it, that's the dreaded 'warm-start' problem! She runs GREAT once we get going!

Fuel accumulator is brand new. With the new accumulator 'warm starts' improved about 50% -- but she runs rough until we rev' up and get rolling!' Before installing the new accumulator, I was lucky to 'warm start' 1 in 5 tries!

Here's the fuel pressure test data (air temp this evening is about 68 F ):

System Pressure 68 PSI = 4.7 BAR

Cold Start Pressure 24 PSI = 1.6 BAR

After 3 minutes @ 900 RPM 24 PSI = 1.6 BAR
After 6 minutes @ 900 RPM 24 PSI = 1.6 BAR
After 9 minutes @ 900 RPM 24 PSI = 1.6 BAR
After 12 minutes @ 900 RPM 32 PSI = 2.2 BAR
After 15 minutes @ 900 RPM 30.5 PSI = 2.1 BAR

Residual Pressure = ZERO -- with the test guage valve open. Valve shut, she holds 4.7 BAR

No change in 2.1 bar pressure at higher RPMs

WUR electrical connection has 12v + with engine turned on. Re-attaching WUR 12v after 15 minutes has no effect on 2.1 bar fuel pressure @ 900 RPM.

I think that's everything. It's been 20 years since I worked on a CIS system and I wasn't that good at it back then!

Your help will be much appreciated!

piscator

[Porsche_monkey]

Isn't the immediate loss of residual pressure usually related to a failed check valve?

[Walter_Middie]

piscator,

I agree that you have a check valve issue.

You are correct that your warm pressure should be up around 3.0 bar. One of the possible causes is low voltage to the WUR. It should be 11.4 minimum with the engine off, and around 14 with the engine running. At 12 with the engine running, you are a little low.

The fuel pump should be supplying 1000cc in 30 seconds. If this is low, it also could effect the WUR. I'd fix the check valve issue, double check for clean electrical connections, and test again before doing anything to the WUR.

[piscator]

Porsche-Monkey & Walt-Middie,

Thanks so much! I'm re-doing (double-checking) the pressure test, as I write this! I didn't disconnect the vacuum last time, so I'm getting somewhat different results. Next, I'll 'check' the check valve!

Walt, WUR voltage is 13v with engine running. I get ZERO voltage with the engine off, w/wo ignition key switched on.

WUR vacuum disconnected, WUR 12v disconnected

Today's Fuel Pressure Readings @ 72 F

System Pressure: 4.8 bar

Cold Start Pressure: 22 PSI = 1.6 bar

3 minutes: 22 PSI = 1.6 bar
6 minutes: 24 PSI = 1.65 bar
9 minutes: 24.5 PSI = 1.7 bar
12 minutes: 26 PSI = 1.85 bar
15 minutes: 26 PSI = 1.85 bar

20 minutes: pressure went up to 2.45 bar - (revved engine, settled back 800 -900 RPM idle, engine temp approx 140 F on hand-held 'microtemp' gauge)

20 minutes: 35 PSI = 2.45 bar
25 minutes: 35 PSI = 2.45 bar

25 minutes: reconnected WUR 12v and pressure went up to 3.35 bar -- (revved engine, settled back 800 - 900 RPM, engine temp aprrox 160 F on hand-held guage)

25 minutes: 48 PSI = 3.35 bar
30 minutes: 48 PSI = 3.30 bar

Residual Pressure: 0.6 -- 0.3 bar held for less than 30 seconds before dropping to ZERO

35 minutes: engine won't restart after shutting down to reconnect WUR vacuum. Need ether to restart, overcome, 'warm start' problem.

With vacuum reconnected and 'ether start'

35 minutes: 46 PSI = 3.2 bar
40 minutes: 46 PSI = 3.2 bar

Any new comments? I really appreciate the help!!

piscator

[ossiblue]

I agree with Rex and Porsche_monkey on the check valve. Your zero pressures after shut down are the major out of spec indicator.

[Walter_Middie]

piscator,

WUR voltage is 13v with engine running. I get ZERO voltage with the engine off, w/wo ignition key switched on

Sorry - the 11.4 volts minimum is with the ignition on, engine not running. At 13 volts with the engine running you are probably OK. Measure voltage at the battery with the engine running - it should be very close to what you get at the WUR (no losses).

You WUR pressures sound much better:

Without Vacuum 2.7 to 3.1 bar
With Vacuum 3.2 to 3.6 bar

Your system pressure also seems fine, but after 20 minutes, it should still be 1.5 bar.

I'd still go after the fuel check valve and see if that fixes the warm start issue. Seems like you have done a good job of checking for issues and have it narrowed down to the check valve.

[piscator]

Walt-Middie & OssieBlue,

Thanks for your help!

I pulled the fuel pump down this afternoon. I'd hoped to remove the check-valve and try cleaning it with some 'gum-out'. Of course, 'Murphy's first law of mechanical repair' had to kick in -- my fuel pump is the Bosch 0580254984 with INTERNAL check-valve !

That means a new pump -- right? Should I replace with the same part # or choose the external check-valve version?

I really do appreciate you guys 'looking over my shoulder' it's fantastic!

piscator

[Porsche_monkey]

You can add an external check valve.

When is a "check valve" not a check valve???

[Walter_Middie]

New fuel pump or not? That's not an easy question. You'll have to decide for yourself based on your circumstances. The pump for my 1975 is no longer available. I have a fairly new one in my car, and a spare. With the cost of these things, and the possible unavailability in the near future, I would fit a more generic pump and external check valve (or just the check valve for now). It will be harder to install, as there will be some fabrication involved, and it won't be stock anymore. However, if it every fails again in the future, you would be ready to get another generic pump and fit it in.

Here's a check valve in the Pelican Catalog for a 944 (your pump is the same as a 79 924): 944-608-951-01-M14

[piscator]

Fuel pump decisions, decisions, hmmmm..............

Thanks Porsche_Monkey! If you hadn't mentioned it, I would never have thought about an 'external check-valve' !

Walter-Middie, I see your point. I guess there are a few things to consider. Still, if the next pump lasts 30+ years, it's not a problem I'll likely be revisiting!

I think your idea to mount a new pump with external check-valve is the most sensible thing to do. I have no trouble fabricating whatever's necessary. Still, I'd really like to get up and running, so if I can get the Bosch replacement locally, I may go that way. Hmmmm.... I'll give the problem a 'big cogitation' and report back

Again, thanks very much for everyone's help!

piscator

[Porsche_monkey]

Somebody said an old CIS VW Rabbit pump is a good cheap alternative. Not 100% sure it's correct....

[piscator]

Porsche-Monkey,

Thanks for the tip! I was just about to sit down and study the link you provided on external check-valve conversion, when I saw your post.

That VW uses the same pump sure wouldn't suprise me. When I "google" the Bosch part # 0580254984 -- I get a match on just about every car out there -- including some version of the Ford Mustang !

Now, back to work!

piscator

[boyt911sc]

Originally Posted by Porsche_monkey

Somebody said an old CIS VW Rabbit pump is a good cheap alternative. Not 100% sure it's correct....

Piscator,

Fuel pumps (Bosch) from VW, Mercedes Benz, Volvo, etc. will work provided you get the correct delivery rate per minute (2000 ml/min). Plus consider the fittings for the inlet/outlet of the fuel pump you chose to buy. I like to stay with Bosch fuel pumps because they are proven for their durability and performance. But the final decision is yours along. There are other brands you could try. Good luck.

Tony

[piscator]

Tony,

Thank-you for your help! I've done a little cross-referencing and (as you say) discovered that a Bosch # 0 580 254 984 should be pretty readily available anywhere you find German or Swedish automotive iron. This pump appears to have been widely used. In a pinch, I suspect you could get some variation that would work from your local Japan Motor or Ford dealership.

Walter-Mid & Porsche-Monkey,

Studying the prices at Pelican and elsewhere, it looks like I could purchase TWO Bosch (internal check valve) fuel pumps for not much more than ONE later (external check-valve) version. Add in the cost of a spare check-valve, 12mm banjo bolt, fittings and an extra 'six-pack' in personal labor costs -- and it comes out about a wash.

I probably won't buy two pumps, but that's how the economics look, to me anyway. I have a couple of local sources that may have this Bosch pump at a decent price. If I can't find one tomorrow, I'll order it from our hosts and probably have it by the end of the week.

Thanks for everyone's advice! This has been lots of FUN. It's the first time I've seriously 'turned a wrench' on an auto in over a decade and having you guys looking over my shoulder has made it a real pleasure!

I'll check in once I get it installed.

piscator

[piscator]

Walt-Middie, Porsche-Monkey, Ossiblue, Tony,

Thanks again for your help! New pump installed. Warm-start problem GONE!

I replaced the internal check-valve fuel pump with an identical Bosch pump (part # 0580254984). I 'lucked out' and found one at a small foreign autoparts store. They charged me $10.00 less than 'internet pricing' -- you can't beat that!

BTW: Our 1978 911SC Targa has 52,000 'on the clock' and this is it's first fuel pump replacement. Before mounting the pump, I took some time to degrease the front suspension components and up under the steering rack. I also wire-brushed and put a coat of paint on the underside of the plate that covers the fuel pump. I figure the best way to prevent rust is not to give it a place to start!

Residual pressure with the new pump held for 1 hour! You guys were right, the problem was the check-valve. Here are the numbers compared with the old pump:

Air Temp 65 F, WUR vacuum disconnected, WUR 12v disconnected

System Pressure: 68 PSI = 4.80 bar (old pump was 4.80 bar)

Cold Start Pressure: 32 PSI = 2.20 bar (old pump 1.60 bar)

Idle 1000 RPM (up 100 RPM with new pump)

03 minutes: 30 PSI = 2.10 bar (old pump 1.60 bar)
06 minutes: 29 PSI = 2.00 bar (old pump 1.65 bar)
09 minutes: 29 PSI = 2.00 bar (old pump 1.70 bar)
12 minutes: 30 PSI = 2.10 bar (old pump 1.85 bar)
15 minutes: 32 PSI = 2.20 bar (old pump 1.80 bar)
20 minutes: 34 PSI = 2.40 bar (old pump 2.45 bar)

25 minutes - reconnected WUR vacuum, WUR 12v, engine RPM 950, temp aprrox 160 F on hand-held guage

25 minutes: 48 PSI = 3.30 bar (old pump 3.35 bar)
30 minutes: 48 PSI = 3.30 bar (old pump 3.30 bar)

Residual Pressure: new pump held pressure for 1 hour

34 PSI = 3.35 bar @ 8:00 PM (old pump 0.60 bar dropped to ZERO in 30 seconds)
22 PSI = 3.30 bar @ 8:20 PM
19 PSI = 1.30 bar @ 8:30 PM
16 PSI = 1.20 bar @ 8:40 PM
15 PSI = 1.10 bar @ 8:50 PM
.05 PSI = 0.30 bar @ 9:00 PM

So that's the 'happy ending'! It may be my imagination, but this P-car does feel a bit stronger with the new pump -- more throttle responsive. I only drove it 'around the block' though -- tomorrow I'll give it a fair test run.

Amusing Anecdote: After getting the P-car 'out of winter monthballs' I've been driving it exclusively. Today, I took our Chevy Silverado to go get the new fuel pump. At the first traffic light, I was certain the Silverado had stalled out.

Annoyed, I shifted into neutral and punched the accelerator. I jumped out of my skin when I heard the 5.3L 'red-line' !! Darn near cracked my skull on the overhead!

I'd gotten so accustomed to the 911's music, I forgot that the Silverado is virtually 'Caddilac quiet' !

Thanks again for all your help! It's been fun 'turning wrenches' with you guys!

piscator