# CIS-E DPR Issues



## mes0cycl0ne (Sep 17, 2011)

The DPR in my '90 Fox started leaking fuel so I sought out a replacement unit (used, of course). The car ran (and will still run) fine with the leaky unit, but here's the mystery:

With both replacement units, the car started up perfectly when they were first installed and the car ran well. With the first DPR replacement, the minute I turned the car off, it refused to ever start again with that unit. With the second, it ran fine for several days and now the same no-start issue.

If I swap my original DPR back in, the car starts and runs like normal: I was stranded yesterday when my replacement suddenly refused to start the car. I swapped the original (dangerous, fuel dripping) DPR back in just to get home and it worked fine. 

I'm thinking this must be a fuel pressure issue as the DPR has some sort of check valve in it, correct? If the pressure were too high going into the DPR, could the leaky one work by venting excess while the good ones cut off flow to the injectors? My understanding of the actual workings of the DPR is somewhat limited.

In this case, what components am I looking at as culprits? Everything else in the fuel system is stock, save both fuel pumps which have been replaced along with various rubber fuel hoses.


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## kwak (Apr 7, 2005)

I don't know anything about the internal working of the DPR. This section of the Mk2 Bentley manual might help - Differential Pressure Regulator. Also see CIS-E and CIS-E Motronic Diaphragm Pressure Regulator and 6.6 Fuel Pressure Tests and Specifications.

Since the engine won't start with either DPR, to isolate the problem a little more you might try two different tests when the engine does not start. Instead of swapping the DPR, try unplugging the electrical connector to see if the engine will start in DPR "limp home" mode. If it still won't start, plug the electrical connector back in and loosen the banjo bolt for the fuel inlet to remove system pressure, retighten, and then see if the engine starts.

Finally, since doing something with the DPR somehow fixes the problem, you can check the electrical signal going to the DPR to see if it is in range, and you can also do the fuel pressure tests.


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## mes0cycl0ne (Sep 17, 2011)

Thanks for the response. I had a chance to tinker a bit and now I'm just baffled.

I basically did what you said: swapped my replacement DPR back in, the same one that seemed to cause a no-start yesterday regardless of whether it was connected. I loosened the banjo bolt on the supply a little and started the car. I guess I was too high from huffing gasoline to remember to tighten it again first. Whoops. But in any case, the car actually started. I cleaned up the mess and connected my DPR test harness and proceeded to set the mixture properly as per Bentley, to -10 mA +/-. It wasn't far off to begin with, really. I tried unplugging the 02 sensor and the reading stabilized at 10 mA, then when reconnected it resumed searching normally. 

Other than a clattering cam follower and a strange knock in the engine, as well as a very rich looking exhaust, the car seemed to be running and idling OK.

I tried starting the car with the DPR disconnected and it fired up in 'limp home' mode as you said, which it wouldn't even do yesterday. I tried repriming the fuel pumps to pressurize the system and restarting and it worked just fine. Now it seems like the car will start right up like normal. 

The only other oddity was a sort of backfire sound in the exhaust when I revved the motor. It wasn't super loud and obvious, but just seemed different than ever before. Could be an exhaust leak that has nothing to do with this at all, I dunno.

Anyhow, sorry for the TMI, TL : DR post. The short version is: what you suggested seemed to work and now, despite having not worked at all yesterday, the same DPR seems fine.

What does this suggest? I'm guessing I should really get a pressure testing kit and go through that process at least. What would cause this intermittent DPR related no-start?


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## kwak (Apr 7, 2005)

I'm hoping someone who knows about what you are seeing while chime in.

Your first post said you experienced non-start with both DPRs so I ruled out the DPR as the problem. But you played with the car which did something. The two obvious places then were the signal to the DPR or releasing the fuel pressure.

The DPR does have enough authority to shut off fuel flow to the injectors. This is only supposed to happen when the throttle is closed and rpm is above 2500. Someone recently posted they had a bad DPR not letting fuel get to the injectors. But non-start happens with both DPRs.

For the fuel pressure I wonder if maybe you are getting a fuel vapor lock and releasing the pressure moves the air bubble. It could also be some sort of fuel pressure problem that releasing the pressure clears up.

Sorry, that is about all the troubleshooting ideas I have.


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## mes0cycl0ne (Sep 17, 2011)

Your insight about releasing pressure/air from the system seemed to be key in getting the 'non-working' replacement DPR to work.

The DPR seemed to be at least part of the issue, because if I swap the leaky original back in it will start for sure (but drips gas all over the place). I thought perhaps the first replacement I got was just bunk, but based on these findings it seems like the replacements are good and something else is going on. 

The vapor lock idea is interesting. The mere fact that a leaky DPR unit fixes the problem seems to suggest that fuel pressure or vapor locking has some bearing on this issue. I'm going to see if I can find a pressure tester or at least someone who can do the proper tests. I may have other leaks in the fuel system contributing to this problem too.

Thanks for the tips. They've ruled out the DPR as the main problem and that's the idea I've been stuck on for awhile.


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## kwak (Apr 7, 2005)

:thumbup: I find bouncing ideas off of people helps.


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## jimbbski (Jun 3, 2007)

Where is our DPR leaking from? I had some DPR related issues after I installed a new engine. The stock DPR did not provide enough fuel to the engine to run right even though the car ran fine on the old engine.
I ended up turning the small set screw to gain fuel flow to the injectors. I ran out of time late last year to test it as the weather turned cold. I live near Chicago. And since this is a race car I will have to wait until April as it looks now to fully road test the settings of the DPR.


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## mes0cycl0ne (Sep 17, 2011)

My original unit began leaking through a number of holes in the plastic body. Some kind of sealant came out in little plugs, but I think the actual failure is a seal inside the unit. I tried replacing the o-rings and then sealed the body of the DPR with epoxy but it still leaks like crazy.

Of course, true to the nature of my Fox, the car is working perfectly now with one of the replacement DPRs and the problem remains an intermittent mystery. Unfortunately I don't have a fuel pressure testing rig to get down to the bottom of this.

I've heard about using the set screw to change the fuel mixture. Since mine is all stock, I've avoided messing with that. I do have some pinging issues (oddly, disconnecting the vacuum line from the advance can on the distributor helps the problem) that have been going on for years, but I don't know if that's a fuel delivery issue or what. So far, it hasn't ruined the motor so I've just rolled with it .


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