Checking system and control pressure on K-Jetronic

I did this fuel pressure check on June 11 and took a bunch of photos to create a how-to thread for this forum. Stuff happens and I didn't get around to it. But here it is.

First photo: the kit as delivered, from eBay or some parts house (I forget which), costing about $60. The first step in this pressure test is to install this kit in place of the fuel line running from the fuel distributor to the warm up regulator (WUR).

Second photo: removing the banjo fitting from the fuel distributor. The white towel is there in case of a high pressure fuel leak.

Third and fourth photo: removing banjo fitting from the warm up regulator (WUR).
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Fifth photo: the pressure tester is hooked up to the WUR. You can see that the Bosch connector that is used to heat up the WUR remains in place. You need to disconnect the Bosch connector while you're testing system pressure or the electric current will be warming up the WUR while you're testing the system pressure and this will make your initial control pressure readings higher than they are at a cold start. The sixth photo shows it disconnected.

Seventh and eighth photo: this blue connector is located on the airflow sensor assembly on the driver side. It's underneath the big black plastic molding that connects the airflow sensor assembly to the throttle body. This is the connector for the fuel pump cut off switch. We need to remove the Bosch connector from this switch or else the fuel pump won't run unless the engine is turning over. And we don't want the engine cranking or running during the system pressure test.

Ninth photo shows the test kit attached, running between the fuel distributor and the WUR. It is important to have the valve located upstream of the pressure gauge.
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Tenth photo shows the valve closed. This setting locks out the WUR and exposes the pressure gauge to full system pressure from the fuel pump. Also, I have turned on the ignition key to the "run" position (III). This starts the fuel pumps running.

The reading is about 78 psi or 5.4 bar. The correct spec for the 400i is 73 to 81 psi or 5.0 to 5.6 bar. So my passenger side's system pressure checks good.

Eleventh photo: the valve is open, allowing fuel to flow to the WUR. What we are measuring now is my cold start control pressure. The reading on the gauge is about 19 psi or 1.3 bar.

Twelfth photo shows that I've reconnected the Bosch connector to the WUR. (I turned off the ignition key first.) With the Bosch connector attached to the WUR, the electric current will heat up the bi-metal arm inside the WUR. This will cause the valve inside the WUR to close, over a span of about three to four minutes. This will cause control pressure to rise, which will lean out the air:fuel ratio.
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The correct spec for control pressure with the bi-metal arm in the WUR fully warmed up and with no vacuum is 2.7 to 3.1 bar (39 to 45 psi). To test with no vacuum, I left the engine off and with the key in the "run" position.

The correct spec for control pressure with the bi-metal arm in the WUR fully warmed up and vacuum of between 0.510 and 0.550 bar is 3.4 to 3.8 bar (49 to 55 psi). To test with vacuum, I simply started the engine and let it run at idle. Correct procedure is to use a vacuum pump, which I don't have. I should have checked my engine vacuum at idle to see if it fell between 0.510 bar and 0.550 bar, but I forgot.

What I did during each of these tests, as the WUR warmed up, was to take a gauge reading every ten seconds and write it down. Here is an abbreviated tabulation of my results, with the readings in bar and the no-vacuum test in the middle column and the engine-running test in the third column:

0:00 -- 1.5 -- 2.5
0:30 -- 2.0 -- 2.8
1:00 -- 2.3 -- 3.1
1:30 -- 2.6 -- 3.3
2:00 -- 2.7 -- 3.4
2:30 -- 2.8 -- 3.5
3:00 -- 2.9 -- 3.6
3:30 -- 3.0 -- 3.7
4:00 -- 3.0 -- 3.8
4:30 -- 3.0 -- 3.8
5:00 -- 3.0 -- 3.8

My warm WUR, no vacuum result of 3.0 bar falls within the spec of 2.7 to 3.1 bar (39 to 45 psi).

My warm WUR with vacuum result of 3.8 bar also falls within the spec of 3.4 to 3.8 bar.

I believe my passenger side WUR is functioning properly.

 

Hi Greg. My car does not have the cold start driveability problem you describe.

With only three days under my belt of wrenching on a 400i, I don't want to go too far into diagnosis or advice. But if your car smooths out after 3 or 4 minutes, this also happens to be exactly the amount of time that it has taken for my WURs to stop increasing the control pressure and to settle into their fully warmed-up levels.

I will draw no conclusions from that coincidence. But if I were you I would get one of these pressure tester kits -- and don't forget also to have on hand new washers for the banjo fittings! -- and see what you get.

The other most basic test procedure is to check for vacuum leaks. I'm going to do that next weekend.

 

Hi Greg. I think the owner of CIS Flow Tech is a Ferrarichat member. Someone else will remember his name.

Again with my limited knowledge I am hesitant to throw out advice. But I do know that the function of a choke on a carbureted car is to enrichen the mixture at cold start. I think the reason why a cold engine won't run well at the normal air:fuel mixture ("lambda" = 1) is that cold fuel does not atomize; it comes out of the boosters in large droplets rather than a fine mist. So you need more of it to get enough combustion to keep the engine running. However, others say that enrichening is needed because the cold engine's tolerances are out of whack, and the parts need to warm up before the piston rings will seal enough to produce proper compression.

Whatever is the reason, a cold engine needs a rich mixture for the first few minutes of operation. What the choke does in a carb car the WUR does in the K-Jet. When cold, the WUR's valve is at its maximum opening. At maximum opening the WUR bleeds off the most fuel pressure. This lower hydraulic pressure, acting on the plunger in the fuel distributor, allows the plunger to rise higher in its bore for a given throttle opening. This causes more fuel to flow to the injectors, enrichening the mixture.

You probably already know this. But what I'm getting to is that your car sounds like it's not getting the needed cold start enrichening. The cause could be that the valve in the WUR is always at its smallest opening, the position where it should be in a warm engine. This pushes the plunger down in the fuel distributor, keeping the mixture at a "lambda" of 1: too lean for cold operation. The pressure test will tell you if this is true. The cause could also be vacuum leaks. These allow additional air to get into the intake stream, downstream of the throttle body, and will lean out the mixture. Looking for vacuum leaks will tell you if this is true.

Or it could be something else.

 

In other news, I did the fuel pressure test on the driver side of my engine yesterday. Here are the results. What we have now in this tabulation is that the engine not running, no vacuum to the WUR results for the passenger side are in the second column. The third column is the engine running, yes vacuum, passenger side. The fourth column is no-vacuum, no engine, driver side, and the fifth column is yes engine, yes vacuum, driver side.

0:00 --- 1.5 -- 2.5 ---- 0.8 -- 1.8
0:30 --- 2.0 -- 2.8 ---- 0.9 -- 2.2
1:00 --- 2.3 -- 3.1 ---- 1.2 -- 2.5
1:30 --- 2.6 -- 3.3 ---- 1.5 -- 2.9
2:00 --- 2.7 -- 3.4 ---- 1.8 -- 3.1
2:30 --- 2.8 -- 3.5 ---- 2.0 -- 3.3
3:00 --- 2.9 -- 3.6 ---- 2.2 -- 3.4
3:30 --- 3.0 -- 3.7 ---- 2.4 -- 3.5
4:00 --- 3.0 -- 3.8 ---- 2.5 -- 3.6
4:30 --- 3.0 -- 3.8 ---- 2.7 -- 3.7
5:00 --- 3.0 -- 3.8 ---- 2.7 -- 3.7

My warm WUR, no vacuum, results of 3.0 bar passenger and 2.7 bar driver both fall within the spec of 2.7 to 3.1 bar (39 to 45 psi).

My warm WUR with vacuum results of 3.8 bar passenger and 3.7 bar driver also fall within the spec of 3.4 to 3.8 bar.

Unless someone corrects me, the fact that the driver side starts out about 0.7 bar lower than the passenger side and stays at least 0.3 bar lower until the third minute of warmup is not important. This just means the right side runs slightly richer for those first 3 to 4 minutes of a cold start. But what is important is that both sides have about the same control pressure -- 3.8 vs 3.7 -- when fully warmed up. That's where the WURs spend 99.9% of their lives.

Also, the system pressure for the driver side is 78 psi. The correct spec for the 400i is 73 to 81 psi or 5.0 to 5.6 bar. So my driver side's system pressure checks good.