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tii fuel pressure check (update 3/27/2011)


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Ever since I installed a new electric fuel pump in my car (July), it has some issues when fully warmed up. Accelerating from a stop light, the car studders like it is missing or the fuel pressure is too low.

Strange thing is it doesn't happen all the time.

Here is the Bosch pump I installed after corresponding with Jim @ Mesa Performance




Today I installed a fuel pressure gauge and observed 24psi at idle which is within the BMW specs.

Owners Manual: 21.7 -29 PSI

Fuel injection manual: 28 PSI

Blue Book states 28.5 PSI @ 4000 RPM



I carefully wrapped the gauge in a rag and went for a drive (hood unlatched). The gauge dropped to about 22 PSI while accelerating, then stayed at about 24 while driving. Is this pressure too low? I didn't check the pressure of my old stock pump, so I don't have a comparison.

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either yer pump is crap? or you have a restriction in the supply

to the pump

crushed pipe, ?

clogged strainer ?

pick up problem ?

28psi is 28psi and that's what it should be at all times

you might also run a volume test

measure how much of a liter it pumps into a measuring

jar in 30 seconds and report back

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Thanks Justin and Uncle C.D. for your responses.

I thought the pressure was slightly off, but wondered just how low the system would operate. Per Mesa, the new pump was originally used on 80-83 320i's and should have provided 70 psi.

C.D. -

In-tank round nylon mesh strainer replaced April 2008.

All flexible fuel lines replaced Fall 2008.

All tii strainers cleaned at the same time.

Tii Mahle fuel filter replaced Summer 2009.

Now my dilemma: Get another one from Mesa, go back to my old stock pump, find a 530i pump (if they are different) or use the E30 style in-tank pump?

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think the purple car has a tii or 530 pump in it if you want to borrow and test before buying a new one Jim, along with an e30 intank pre-pump. pretty sure i won't be using them for a while. s14 ran fine with the pumps before removal.

i assume your a/f gauge is showing leaning during accel where the fuel pressure is low?

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My experience. When I bought my Turbo, it was unable to be driven above 4.000 rpm. Additionally, the fuel pump was very noisy and leaking. I replaced the fuel pump for a 5 series one (same than yours) and the inline gas filter. Situation was even worse to the extend than the car stopped in the midle of the road.


Then I replaced the nylon filter inside the tank and everything was fine for 10,000 km. Then noises appeared again and the car had some hesitations above 4.000 rpm. That time I mesured the fuel flow. Fuel flow was 1 liter per minute. I replaced one more time both the in tank and inline filters. Once they were replaced the fuel flow was 2 liters per minute. In both cases I mesured the flow at the tank return hose.




One more time the car run ok, but after 500 km hesitations above 4,000 rpm came back. That time I replaced the fuel pump with a pump scrapped from a e21. After 5,000 km everything seems to go well.

Regards from Spain: Pepe de la Parte.


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Thanks for more worldly experiences - sorry your car has had multiple issues.

I connected the pressure gauge before the aluminum canister filter - same 24 psi results. Hard to install it since my battery is still in the front.

Disconnect and remove battery

Remove fuel line from inlet of filter and connect T from gauge kit

Connect gauge to T fitting

Re-install battery

I didn't do a volume check since the pressure was too low. Time to install yet another pump.

Marshall - the A/F gauge did indicate a lean condition when the stumbling occured.

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the new pump was originally used on 80-83 320i's and should have provided 70 psi.

err, if they said that, they were wrong. The CIS pumps are supposed to

deliver somewhere in the 20's, just like you are measuring.

It's a bit more money, but if you want to do it once, get an EFI pump

from something with electronic injection and quite a bit of flow

(Ford Mustang in- tank, whatever you want) and then regulate it

down to 28 psi. It's overkill, but then you'll be sure. Summit can sell

you an adjustable regulator for that range- it's meant for TBI cars.

Along the same lines, a TBI pump should be close to the right pressure.

Probably overkill- I didn't realize the tii was so fussy about inlet pressure.


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Yep, he said above he measured 22psi while driving. Tii register site sez:

Check your fuel pressure: Low fuel delivery pressure causes fuel starvation at high speed, full load operation Fuel pressure deterioration has several causes.

First, check the electric supply pump for proper delivery pressure. Disconnect the fuel supply line to the injection pump at the inlet banjo. Test delivery pressure with the ignition on and the engine not running. Pressure should exceed 50-60 psi. Low pressure indicates problems with the supply pump. Excessively high pressure indicates that the pump pressure relief valve is not operational. Correct as necessary.

Second, check operation of the pressure maintenance valve. Re-connect the fuel supply line the pump banjo. Connect a pressure gauge to the fuel supply at the cold start connection and turn the ignition on with the engine off. Line pressure should not be less than 28 psi. Start the engine and blip the throttle to 5000 rpm. Fuel pressure should not drop below 25 psi.

Third, check fuel supply volume. Disconnect the cold start fuel line and turn the ignition on with the engine off. Measure the volume of fuel delivered into a calibrated container. Volume should be (pick one) 29 gal/hr, 1.83 liter/min, or 0.46 liter/15 sec.




have you measured the presure while you drive ?
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