Poor mileage

[Zero10]

I have an 86 944 NA, and I only get 400km to the tank, if I am lucky I get 420km. This is FAR below what I should get, no? so I started checking things. Started with a few misc things, O2 sensor, fuel filter, nothing yet. Changed them both and it only got worse (useta get 480-500km). I checked the fuel pressure today, getting 36-38psi running, 40psi jumpered, and the leakdown (20 mins) it only lost 4 psi. (36 down to 32). I don't know what to do next. Could this be due to my pressure regulator? Although it does open at 40psi, I can hear SOME gas pouring though the tube back to the tank. Could somebody tell me what to check next? This is getting very expensive, I have seen turbo 944's that go 650k to the tank, and gas is expensive, so I need to fix this!

[Zero10]

I understand what it should be. At what point does the regulator open? does it open gradually, or at say 36psi, does it just pop open? I was also wondering if very dirty/plugged up injectors could induce a high pressure at the rail? Although I doubt they would raise it that much. Is there anything else I should check?

[Porsche944]

The regulator is designed to use vacuum to oppose spring pressure to maintain fuel pressure. If vacuum to the regulator is low (like when you jump on the throttle), fuel pressure goes up in anticipation of an increase in air flow.

If you have a vacuum leak, vacuum to the regulator will always be low and fuel pressure will always be high, resulting in poor gas mileage.

First off, check vacuum at the regulator. You should have >14" Hg with the engine at idle. This vacuum comes from the rear most port on the throttle body. If the vacuum is low and the lines are good, chances are the throttle body is out of mechanical adjustment.

To check the mechanical adjustment, remove the throttle body, loosen the Throttle Position Switch hold down screws, loosen the mechanical stop jam nut, and then back out the mechanical stop until it does not touch the stop arm.

Clean the throttle bore as necessary. Make sure that both vacuum ports are open. One port is above the throttle plate and one is below. Check throttle plate movement. The throttle plate should go fully closed. Once you have this, turn the mechanical stop until it just touches the stop arm, turn the stop screw and additional 1/2 turn, and then tighten the jam nut. This sets the throttle plate mechanical stop.

Now adjust the TPS so that you hear a faint "click" just as the throttle plate comes off the seat. Tighten the TPS hold down screws and reinstall the throttle body.

Connect a dwell/tack to the coil, and connect a vacuum gage to the line on the regulator. Use a "T" fitting so vacuum is applied to the regulator. Connect a fuel pressure gage to the fuel rail. Jumper pins B and C at the diagnostics connector on the driver side fender will.

Start the engine and let it come up to normal temperature. With the engine up to temp, adjust the idle by-pass on the throttle body for 840 +- 50 RPM. Remove the jumper at the diagnostics connector. If the idle stabilizer is working properly the engine idle should stay within spec.

Now you can check fuel pressure vs vacuum.

[bearone2]

are these regulators adjustable for the 29 psi.

where is it recommended to tap in to check the pressure.

[mike944]

Not sure if this applies to 944's or not, but on most cars, you disconnect the vacuum line to the fuel pressure regulator (FPR) when you check operating pressure. The reason for the vacuum hose connection to the regulator is not to increase pressure in anticipation of increased airflow, but rather to keep the pressure differential across the injector constant (i.e. rail pressure minus manifold pressure = constant) during high vacuum, the manifold is approximately negative 14 psi and the rail is 29psi (taking FR Wilk's word on this one) therefore the pressure drop across the injector is 43 psi. at low vacuum, the manifold is approximately 0 psi, and the rail SHOULD be around 43 psi keeping the pressure drop across the injector at 43 psi. If you were to pull the vacuum hose off the regulator, the rail should always be at 43 PSI, regardless of manifold pressure.

FR - is that 29PSI regulated, or unregulated (i.e. vacuum line connected, or disconnected?)

Mike

[Porsche944]

I beg to differ. I have connected a hand vacuum pump to the FPR fitting and a vacuum gage to the fuel rail, and fuel pressure changes with vacuum, PERIOD.

If you knew ANYTHING about internal combustion engines you would know that when the throttle opens manifold vacuum drops initiallly and then increases with engine RPM.

During the initial vacuum drop the engine will run lean unless extra fuel is being metered. HINT: Ever hear of an "Accelerator Pump" on a carb? Having the fuel pressue increase with a change in vacuum does the same thing, it corrects the inherent "flat spot" that ALL reciprocating internal combustion engines have. Injector differential my butt.

For Zero10: The stock FPR is not externally adjustable. Pressue at the fuel rail is a function of vacuum to the FPR (More vacuum=Less pressure) and the max working pressure of the fuel pump.

The best place to tap into the FPR line is at the FPR. Get a short piece of hose and a vacuum "T" fitting. One side of the T goes to your gage, one side to the FPR, and the other side goes the the vacuum line from the throttle body. Checking fuel pressure is another story.

There is a nice threaded fitting with a cap at the end of the fuel rail. However, the fitting at the end of the fuel rail is metric, so your run of the mill fuel pressure gage won't work. What I did was cut the schrader fitting off of my gage and then I used a hose clamp to keep the hose connected to the fuel rail.

A real easy way to test this system is with a hand vacuum pump. Connect the pump to the FPR fitting and plug the line going back to the throttle body with a golf tee (You still need vacuum to the damper). Start the engine with 0" of vacuum at the FPR. Take note of idle RPM and any odd vibrations or shaking. Slowly increase vacuum to about 18" or so. As the vacuum increases the engine should smooth out as the fuel/air mixture goes lean. This tells us that the fuel pump and the FPR are doing what they are supposed to do. If you cannot get or maintain vacuum to the FPR then chances are the FPR is bad.

[mike944]

Yes, the fuel pressure changes with vacuum! But the pressure differential should remain constant, that is, of course, if the FPR is a 1:1 ratio (most cars are 1:1, although there are exceptions) If the 944 is a 1:1 FPR (i'm not sure it is) Fuel pressure should decrease by approximately 14 psi when going from no applied vacuum to full vacuum (30" Hg)

I do know plenty about internal combustion engines, yes, manifold vacuum drops with throttle opening, and rises again with increased rpm (increased flow) and i NEVER said anything contradicting this.

During the initial vacuum drop the engine will run lean unless extra fuel is being metered. HINT: Ever hear of an "Accelerator Pump" on a carb? Having the fuel pressue increase with a change in vacuum does the same thing, it corrects the inherent "flat spot" that ALL reciprocating internal combustion engines have. Injector differential my butt.

Hello....Extra fuel is metered by the computer, that's it's job. Does the term "injector pulsewidth" mean anything to you? This fuel injection parameter known as "acceleration enrichment" (also known as other names) is adjustable by the programmer of the ECU. This additional parameter is applied during rapid opening of the throttle to cure the flat spot you're referring to. How many fuel injections systems have you programmed?

I wasn't trying to make an argument out of this, we both seem to agree that fuel pressure goes up as the throttle is opened (manifold vacuum is reduced) we just disagree on why.

Mike

[Zero10]

Okie dokie, I seem to have lost my vacuum gauge, so I cannot test it at the FPR, However when I pull the line off, there is no difference in RPMs or how the engine seems to run (I already had the pressure gauge off, so I don't know what the fuel pressure did), and I stuck my finger on the line, feels like there isno vacuum there. I traced the line back to the throttle body, and when I pull it off there, it seems to have a strong vacuum, but when I trace it through all the tee's and everything, I cannot find a split or break in the line. I don't know why it has no vacuum.
Perhaps this is where my search should start?
On a side note, there is a flap attached to a wheel inside hte throttle body, the vacuum line for this comes off BEFORE the flap, this should not give very much vacuum at all. Why is it like this?

[Porsche944]

Zero10:

Remove the existing vacuum line completely (Including the section that goes to the damper) and give it a good visual inspection under a strong light. The vacuum line is plastic and can develope cracks that are hard to see, so take your time.

The fact that you have a strong vacuum at the throttle body says that either the line to the regulator and damper is leaking, or it is clogged.

If the plastic tubing checks out OK, try securing the rubber hoses to the tubing with small nylon tie-wraps. I used this trick on my former '84. I gained 4" of vacuum at the FPR, and had similar results with my current '87.

As for the other port on the throttle body, that port is above the throttle plate and only has a small vacuum signal (about 5" or so) just as the throttle plate comes off of the seat. This signal is routed to the emissions control valves on the driver side fender, and tells the system to purge the charcoal cannister and fuel tank.

[Porsche944]

For Mike944:

OK, time for me to pull in my horns. Some times my buttons get pushed, and I type before I think. My issue was Zero10 is obviously a "new-bee" who is stuggling to get his car running like it should.

In cases like this I try to offer some encouragement and keep the technical stuff as simple as possible so the new-bee doesn't get off on a wild goose chase.

When you get right down to it, cars are nothing more than a bunch of simple systems that "conspire" to make up a complex machine, and often times we get caught up by these complexities when the asnwer we are seeking is simple and straight forward.

In Zero10's case the symptoms point to a problem with the FPR not controlling fuel pressure, which has two causes: Bad FPR, or incorrect vacuum.

Zero10 had obviously done some homework as incicated by the numbers he was using. The problem was he didn't know what the numbers meant, or how those numbers came to be. The fact of the matter is, those numbers aren't really germane within the context of the problem.

What the numbers should be telling us is that the fuel delivery system is operating properly based on the control signals that are being applied to it. The next step in this process is to determine what, if any, vacuum is being applied to the FPR, and Zero10 says there is no vacuum at the FPR, but there is vacuum at the throttle body. OK, fine. Now we have moved on to the vacuum lines, and I'll wager several cold ones that Zero10 finds a broken line.

My point here is that with the exception of mentioning a minimum of 14" of vacuum at the FPR, and the idle RPM spec of 840 +- 50, I didn't use any numbers because there was no need. Zero10 doesn't need to know about injector differential, manifold vacuum, or fuel pressure, he just needs to get vacuum to the FPR.

Anyway, sorry for the tirade.

[Zero10]

Not to be rude here, but I'm not a new-bee, just new to fuel injection, I have torn every car I have ever owned to pieces, and put em back together again, better than before. The reason I am asking such stupid questions is because the simplest answer is usually the correct one. Replace the FPR is not the simplest answer, vacuum leak is. I have checked all the hoses visually, and they all appear fine, felt them all over (not in that way =), and they are still soft, I can't find a crack anywhere. I am going to try replacing that set of vacuum lines anyways. My new question is, the line tees off almost immediately, leading up to something above the diagnostic plug, by the base of the hood, driver's side. I do not know what this is. However, when I pull the vacuum line here, I can see no difference in anything, but I do get a strong vacuum at this port. So, long story short, what the hell is this thing, and could it be broken inside, causing my loss of vacuum on this line?

On a side note, it tells it to purge the charcoal canister and the fuel tank?.. First of all, I sure hope it doesn't purge the fuel tank *grin*. second, I am starting to think the vacuum line routing on my car is incorrect. What does the charcoal canister do anyways? I know it is related to emissions, but that's it.

[Porsche944]

Zero10:

OK, so you're not a true "new-bee" to cars, just to Porsche fuel injection systems.

There are two vacuum circuits for the emissions system. The "weaker" vacuum controls when the system purges, and the stronger vacuum does the actual purge.

The charcoal cannister collectes and stores un-condensed fuel vapors from the fuel tank.

Both of the valves in the cannister purge system can develop internal leaks, so if your vacuum lines look good, just plug the lines to the emissions purge valves. You can run the car this way without doing any harm, but you may not pass your local emissions inspection (If you have one that is.). Just make sure you have vacuum to the FPR and damper, and you should be OK.

BTW: I believe that the only stupid questions are ones that never get asked.

[Zero10]

Hmm, upon a second inspection, it would appear that my hands were simply too cold to feel that there was indeed vacuum at the FPR. I will get my gauge to measure it. Perhaps I still have a leak.
If I have good vacuum at the FPR, and my pressure is still 10psi over at an idle, should I replace the FPR? I'm not sure, because it is almost normal when jumpered. Perhaps the vacuum line inside it is just plugged?

[Zero10]

Alrighty, got out a vacuum gauge (ANCIENT!) and I get 14" Hg at the FPR, just to make sure my gauge wasn't hooped, I checked the vacuum at the manifold, I get 13" Hg, so whatever I unhooked caused the engine to run worse. Since I have 14" Hg at the FPR, and 36psi fuel pressure, does this mean my FPR is messed up?

[todwic]

bad FPR.

• pressure is supposed to be @29 psi
• When you unhook the vacuum to your FPR, the pressure should jump to the pressures that you're seeing. I don't have a longwinded reason why.
• If you want to test your pump, clamp your return line. pressure should shoot up to over 100psi
• I connected a female air chuck to my fuel rail. male air chuck to the line leading to my air gauge. The female seals the fuel line in case you want to tool around the neighborhood for a test drive. DO NOT SPILL ANY FUEL ON YOUR DISTRIBUTOR. bas stuff there

You should buy a manual of some type if you want to effectively trouble shoot your injection system.

[Zero10]

yeah.. I made a mistake plugging my end cap after testing the pressure, shot about 5 litres of fuel before I could shut it off, fuel pump is definately strong, lucky it all missed the distributor, or else I would be posting about something completely different. I think my FPR is bad as well, seems like it's not responding to the vacuum, however the car sat for 11 months before I bought it, is it possible to restore a FPR? A replacement is 100 bucks (CDN), which isn't too bad, but if I can fix it that's even better.
I understand that the FPR opens gradually, and under a vacuum, with the injectors firing, it should read about 29 psi, now my injectors are pretty plugged, so I could understand upto maybe 32 psi, but 36-38 is getting excessive. And whe it's jumpered, the reason the pressure is 40psi isbecause there is no vacuum, and no injectors, correct?
So, the FPR opens gradually, based on the vacuum applied and the rail pressure. I hope I got that all right.
I will order a new one tomorrow, I'll keep posting about whether or not it fixes my problem.
On a side note, when I induce a vacuum leak, my car starts better, lol. It normally cranks 6 times then fires (6 every time, dont ask me why), with the vacuum disconnected (not capped) from the FPR, it only turns over once before catching. Hopefully the new FPR will work like this.

[todwic]

There are two parts to testing FPR (This is from a Bosh manual
• Engine idling: 29psi. This test regulation of pressure by the regulator
• disconnect the fpr and block off the vaccuum line. because the regulator senses higher pressure that manifold pressure, the pressure should jump up to 36 psi.
• When you reconnect your line, the pressure should return to 26 psi. If it does not, your fpr is bad

No, you can't fix your fpr. replacement is the only option.

• That is: provided that all of your vacuum lines are intact. If you have a vacuum leak, your pressure will read 36 psi regardless. Since lines are cheap, and do go bad, I recommend that you replace those first.

[todwic]

I think that your car starts better when you induce the leak because you are allowing extra air to offset the xtra fuel your engine is getting.

[Zero10]

That's what I think as well, I guess I'm not crazy =).
It seems like my FPR is not reacting to the vacuum at all. I will order a replacement today, but before I install it, I am going to re-install my pressure gauge, and try unhooking the vacuum line with it on there, see if the pressure goes up (I strongly doubt it will), just to make sure mine is hooped. The reason I asked if I could fix it is, the car sat for 11 months before I bought it, although that should have nothing to do with the vacuum side of it, the fuel rail was dry too, so perhaps it dried up inside...
Oh well, gonna be out 100 bucks soon, hope this fixes it!

[Zero10]

I heard on another board that a 36-38psi fuel pressure will not cause my bad mileage, is this accurate?? It seems to me that this should give it about 1.3-1.4 times the fuel it thinks it's getting, but I was told the computer, based on O2 sensor readings, will correct this by reducing the injector open time. Is this true?
I will be measuring my O2 sensor voltage today, to see if the computer can balance this out.

My plugs are a rust color, they are not very sooty, a little black on the edges, but not very black like they should be. Have I just done all this work and not found the problem? What else could cause my bad mileage?

This may be important, my compression numbers are all between 115psi and 125psi, this sounds a little low. Would this cause my problem?