'79 Bosch CIS fuel injection control pressure to high


Asked by May 18, 2012 at 01:37 PM about the 1979 Porsche 924

Question type: Maintenance & Repair

  I have a '79 924 Porsche which used pretty much the same CIS injection as my 81 & 82 VW Rabbit convertables. Control pressure is only about 4 LBS's less than my system pressure of 75 LBS, and car won't rev up at all.  My Rabbits run ok so switched out the warm up regulators with them but no change, wur fuel lines are clear, return fuel line is clear, the plunger in the fuel distributor is free. Don't know what else to check other than replacing the fuel distributor. Help

21 Answers


your warm up regulator is clogged or out of adjustment i'm dealing with exactly the same thing with my 77 adjustment is done by moving the seell plug in the WUR you will need fuel presure guages to know where to adjust it to and there is a modification that can be done to make adjusting way easier just drill and tap the plug for a screw and a nut to push or pull the plug where you need it to go good luck


Can't be the wur out of wack cause in the above post I said I tried the control pressure regualtors (wur's) from my two Rabbits and no change, Rabbits run fine. I also had bought a new wur and no change ,even sent it back thinking it was defective for another so I've had a total of 4 wur's in the car now and no improvement. So still scratching my head over this one, guessing its a bad fuel distributer but only guessing. Yes I have tested it with a fuel pressure gauge thats why I know the control pressure is to high.


let me get one thing straight u placed the original CIS injection with less power in your 79 Porsche from the VW rabbit ! if that is what u did i think because of the low pressure it is creating lean mixture in the cylinder which is causing the power drop Hope this helps

1 people found this helpful.

ok then the plunger could be stuck in the fuel dizy or the little rubber "O" rings could be all cracked or the diaphragm could have holes in it time to take it off and get a good look at it if the plunger is stuck boil the dizy in water for an hour or so and it should come out with small vise grips on the nub .if thats free then you will have to take the top off and look inside there are good write ups on fuel dizy rebuilds on rennlist in the 928 forums good luck


Like I said in my first post, I checked the plunger and it was ok but haven't gone inside the fuel distributer any frother as the book says not to but might have to do it. Do you know who carries rebuild kits like the diaphrame etc ? Wonder if anything inside the fuel dist. can cause my very high control pressure readings ?

1 people found this helpful.

Muhammad, I put the wur from the Porsche into the Rabbits and it works in the Rabbits and I put the working wur from the Rabbits into the Porsch but it didnt work in the Porsche so that pretty much tells me that the regulators are all good.

Gene321, I think I read about your problem on the 924 board as well. If you're still looking at cargurus, I also have a '79 924 with the identical problem. I also think the problem is my distributor. Did you replace your distributor, and if so did that correct the problem? If my WUR has failed, and unless it's just plugged up, I can't comprehend how it could deliver anything but either a too rich mixture continuously or the leaner mixture for normal/warmed up operation.


Yuperglenn, Have you had any luck with your CIS ? I checked for high or low pressure along the fuel curcuit . Have low pressure to wur, and low pressure out of wur so wur should be ok but I have high pressure from the wur line I removed that goes to the 3 way bango on the fuel dist. So I removed that banjo from the fd and I have high pressure comming from the threaded hole. Wondering if this is nornal ?? This could be where the problem is comming from , in the f d

Gene, no luck yet. Haynes states the you should have 20 ohms of resistence across the spades of the WUR. I did check that a week ago and the value I got was exactly 20 ohms. Regarding your question of high pressure from the threaded hole, I think you are on to something. Is that controlled by the "system pressure regulating valve" - that hexagonal plug on the corner of the fd, as Haynes describes it? I have removed that plug, the shims and needle. I reinstalled the needle and plug leaving the shims out and for some reason, dropped system pressure 10 psi and picked up 500 rpm. I don't know that I understand why , but I'm thinking that your high pressure coming from that threaded hole is working against what the wur is supposed to do. I do think that with the exaustive swapping of wurs back and forth from your Rabbits that - well, in my wildest dreams, I can't imagine that your wur is bad. I think it's your fd and the same in my case. What bugs me is that the only fd rebulid kits I've found consist of a bunch of copper washers and a new system pressure regulating valve needle, tiny o-ring, shims, etc., but no internal fd o-rings, diaphram, or any of those parts. So I've been reluctant to take it apart. BTW, my car sat a ton of years. I had the fuel tank renewed, replaced both fuel pumps and accumulator, purged high pressure and return lines, replaced fuel filter (twice), injector sockets, o-rings and injectors. I have 70 psi between the filter and fd and the primary pump is drawing 6.8 amps. But i only have 425 cc coming from the return line in 30 seconds. Haynes says it should be at least 750 cc. and more like 6.5 amps. My plunger has no sticky spots and moves freely as well. The most I can get is about 2500 rpm, 3000 if you nurse it, but it's super lean (white inside the exhaust manifold). Next week I'm going to pull the injectors and look at pattern and consistency. It could be that gunk from the fd has partially blocked them and I may have to replace them again. After that I think the fd is coming off. Later...


Glenn, Yes I can see why your system pressure would change if you removed the shims as by putting different thickness shims in changes the system pressure. But something else is screwing up the control pressure so you need to keep the system pressure at a correct pressure. Yes I agree I think the problem is in the fd. I'm still working with a guy from the Palican fourm (e mailing) who is working on the problem but I think I have him stumped as I"ve not heard from him in awhile. I did find a company that rebuilds the fd but I don't trust them as I bought a rebuilt wur from them and after a year they still have not returned my core refund. And I think the core refund for the fd is $100 . If they wouldn't have screwed me I probably would have gotten a fd from them. I did see on the Palican site where a guy on the 914 forum did rebuild the fd and his car ran find afterwards so it can be done, might want to check out that thread. Me I don't know if I have enough balls to pull mine apart LOL When I did my volume test I had the correct amount of fuel in the 30 second test unlike yours. But that was a year ago so maybe I should retest. I tried to get on the 924board.org to talk to you but can't connect there any more for some reason. In case we lose our talking here, here's my address starman3286@yahoo.com as I'd hate to lose connect with you as we seem to have the same problem.

Well, I had a little set back. I was preparing to retest my pressure and flow volumes, with and without injectors. Since the car had sat for a while, I topped off the battery because I wanted the pumps to operate at the correct speed or the results wouldn't be valid. Haynes cautions one to disconect the alternator connector or battery cables before charging the battery so as not to damage the diodes in the alternator, however this time I forgot. I think I smoked the alternator. I had no power to the fuel pumps (when jumping 30 and 87 at the starting relay) so started looking for the problem. The heavy wire from the alternator to the starter (actually there are 2 wires about #12 size each in one insulation) got toasted from one end to the other and melted off near the starter. The battery was down a volt and I charged it for 40 minutes, starting out at about 7 amps. The diodes appear to be okay, but the alt could be otherwise damaged internally. I removed the exhaust pipe from the cat forward, the oil filter and related heat shields to actually see what had gone wrong. Not wanting to repeat this in the near future, I did order a new alternator. I will have the old one tested anyway. Do you think charging the battery with the alternator caused the wire to melt? I'm not convinced of that but want to be pretty sure that's the reason before installing the new alternator. Yuperglenn


Guess I never read that chapter in the Haynes as I've always charged my battery without disconnecting the battery and nothing bad ever happened to me. Sounds to me like something touched that shouldn't have like a heat shield to the hot wire or maybe the wire to the fp touched metal ??? I had a misshap on mine also, when I plugged the fp relay back in last time the hot wire got pushed out of the relay socket and touched a ground wire and the car filled up with smoke, funny thing is I can't find any burnt wires any where so think they are hidded somewhere in the wiring harness, scarry !

Gene321 Yes, I really don't think charging the battery was the problem. Chapter 11 sections 4 & 6 caution about charging with the battery connected to the system but...I've never had a problem withthat in my life. I've checked wiring around the FPs, as you mentioned and that all looks good. Plus I think at that point, it should blow a fuse. My theory is that when I was re-attaching the the bell housing/drivetrain/exhaust system, I may have unintentionally moved the alternator wiring to where it shorted to ground. There was a spot where the outer sleeve had worn a little hole in it where it was rubbing against a heat shield (as you had mentioned) which is the most likely cause. I was thinking "why just that wire? It's only 12 or 14 inches long and consists of what looks like two #12 wires inside one insulation. But it goes between the solenoid and the alternator and attaches at both ends to I believe 8 mm studs - pretty healthy ones, so I think it had to have grounded out somewhere between the two. I had the alternator tested and it failed. They said the stator was bad.

Gene321, Well, I got the electrical problems repaired. The old alternator failed testing - bad stator. It may or may not have been due to the short. In any event, I replaced the alternator and wiring. You were right about the short. Upon inspection, it looked like the insulating sleeve (which is really quite nice stuff) wore a hole right at the front edge of the starter and grounded out. Anyway, that's taken care of. Oh, in chapter 3, Para 15.2, they also caution that when running the pump (as in testing) without running the engine, one should also disconnect the alternator plug, right at the firewall in front of the battery. I never had a problem withthat either but it's easy to do. Back to the FD. I ran some more flow tests - pulled the injectors and put them in individual containers. I ran the pumps for 2 minutes at the equivalent of WOT (with air cleaner removed, I gently propped the fuel metering valve "lever" all the way up - which is about an inch) and measured the fuel delivered to each cylinder with a beaker. I did this test a total of 7 times, with and without injectors on the lines at WOT. The results were similar to last January when I did this. With injectors on the lines I averaged 120 cc on cyl 1 and 2 but only 85 cc on 3 and 4. After removing the injectors, I averaged 140 cc on cylinders 1 and 2, but only 89 on 3 and 4. Actually with and without injectors on, 1 and 2 averaged 140 and 120 respectively, but 1 was 10 cc higher and 2 was 10cc lower, whereas 3 and 4 were very close to the same with and without injectors. Haynes said to "lift the sensor plate slightly" for this test, but the above would be at WOT. So, I propped the lever up at the equivalent of 1/4 throttle and ran 3 more tests. Amazingly, all 4 cylinders delivered between 75 and 80 cc consistently in all 3 tests, without injectors on. Makes sense - it runs okay but starts flattening out by 2500 rpm. It has to be the FD. Why else the difference? I can't see why anything else would cause this - certainly not the WUR. One could refine numbers by testing at 1/4, 1/2, 3/4, and WOT, but I had enough. Ultimately the FD dictates how much fuel goes to each cylinder. I was thinking about removing the coldstart valve, disconnecting the return line right below the master cylinder, removing the fuel line from the output side of the fuel filter, plumbing up a tank sprayer that I can regulate to 70 psi, and run some laquer thinner through the FD and associated fuel lines in an attempt to clean it, but if the diaphram is bad it would be a waste of time. And the fuel lines might not like it. I called Jaytan Industries, Inc. and talked to "Terry" in tech support. I reviewed the above numbers with him and he felt sure it was the FD. Think I will send it to them. Testing will cost $50. If I have it rebuilt, it will cost $212.99 minus the $50. That way I will eliminate the $200 core charge hassle.


I did all the tests also you did and had the same results so I agree the fd must be at fault so I will probably send mine to Jaytan Industries. also at some point this winter. Another thing that puzzles me , last year my 924 and VW pickup both brokedown within a week of each other. The VW when cranking the motor over the first cold snap here in Co. sounded like the plugs were removed, turned over fast etc and wouldn't start below 60 degrees so I pulled the pan and replaced the rings. That improved my compression from 50 something lbs to 70 lbs but still cranked over like no compresson and wouldn't start so I pulled the head and had the valves ground which brought up my compression to 90 something but on cold mornings it still wouldn't start. Now since I drove it some last summer it started right up when it was in the 20's the other day.Scratching my head over this one LOL

Gene321, Wow, your VW pickup issue is certainly amazing! In my wildest dreams, I can't imagine what could be causing that kind of spontaneous change. OMG, I mentioned that I had gotten my electrical problems repaired, but I hadn't started it yet. Turns out that when I started it, it wasn't charging. I assumed I had more wiring issues and spent a bunch of time checking all of that out. I finally removed the new (reman) alternator and made up a test bed in order to make certain that it was good. I even included an idiot light and fuse in the exciter/ffield supply line with little jumper wires. Guess what! The alternator was no good. So the local auto supplier got me another one, and I tested that one before installing it. It was good and the car charges now. So the electrical problem is behind me now. I know one can get a bad alternator (or anything else), but it doesn't happen very often. I just happened to be the unlucky one. Funny story. Actually, there are two auto suppliers here - one can test and the other can't. I had the OEM alternator tested at the NAPA store and it failed. They wanted something like $160 for a reman one, and the Auto Value store got me one for $95. When I started suspecting that the reman alternator might be bad, I didn't have the balls to have it tested at NAPA, so had to make my own test bed. Funny how circumstances can but one in the butt sometimes! Okay, so I removed the FD last weekend and sent it to Jaytan Monday. They received it Thursday, rebuilt it, and shipped it out to me Friday. I should have it this coming Tuesay or Wed. It won't take me long to stick it on and I'll let you know how it runs. Glenn


Glenn, So did the rebuilt fd fix your problem ? If so I'm going to pull mine and have it rebuilt also. Gene

Gene, the short answer is yes - it hasn't run like this since I've owned it. If you're interested in the long answer - well, the following narrative is amazing! Seriously, I struggled with this, mostly because of a bit ignorance (never worked on CIS before). I got my FD back, the piston moved freely and everything looked good. I installed it and the engine wouldn't run at all. I called them, and they were very apologetic - said they would send another one to me, and I received that in a few days. I installed it, started the engine, and it ran perfect for 10 - 15 minutes. I didn't red line it, but took it up over 5,000 - no problem. I wondered about a hot start, so shut it off and restarted it - perfect! Boy, this is great! Well, the next day I started it and it ran crappy again - probably worse than ever. I couldn't believe it! So I called them again. They were having trouble figuring out what was going on but said they would again go through the original FD I had sent them (which they had originally rebuilt and I had returned when it didn't work) and send that back to me. I will probably get that one tomorrow. In the mean time, (today) I have gotten the engine running well again on the second FD. Now for the rest of the story. I think I now know what happened in this string of events. I may have screwed my original rebuilt one up. When I removed it because the engine wouldn't run, the piston was stuck all the way up - couldn't get it down. Also, prior to removing it, fuel had been running out the bottom of it, into the air box, effectively washing any old residue that may have accumulated on the sensor arm/pivot point/adjustment mechanism. More on that later. When I installed that first FD, I put a block under that arm in the air box to make sure the piston wouldn't drop off to the side of the arm when positioning the FD. Before tightening the screws, I should have removed the block but didn't. I've used that block for flow tests in the past, but this time may have located it farther under the sensor arm, raising it higher. Anyway, I think I'm responsible for the piston getting stuck all the way up, and there probably wasn't anything wrong with their rebuild. One other caveat: when I conducted all those flow tests in the past, I referred to WOT and 1/4 throttle. That sensor plate arm has approximately 1 inch of travel. All the way up (what I was calling WOT), I was getting approximately 100 cc in 2 minutes prior to rebuilding. On the (second) rebuilt FD I measured close to 250 cc in one minute. Man, that's a liter/minute. I don't think a full size truck uses that much fuel. All this is to say that the sensor plate may only move 1/2 an inch from idle to full rpm, certainly less than full travel. - so one needs to be careful not to force that piston up too high. In my case, I think I messed up when I initially installed it - stupid hurts! Back to the 2nd FD - worked and didn't work. I have never messed with that adjustment screw, under the rubber bung between the air sensor boot and the FD. The quick answer is that after some trial and error, about 1.5 turns clockwise (richer) made a huge difference and it now runs fine - starts good and all. What happened? There has to be a reason. I didn't remove the air sensor unit or disassemble any of it. But that 3mm hex adjusting screw must slightly change the mechanical advantage of the arm acting on the FD piston, or something like that. Either I mechanically disturbed that setting when installing the first rebuild or all the fuel running out of the first FD (because the piston was up too high) bathed that mechanism so that it spontaneously changed (relaxed) the setting between the first and second time I started it. Through it all, Jaytan has been very cooperative. They are approachable, and if they don't answer, they will return your call. My experience with them is very good. If you have yours rebuilt, you may have to make some fine adjustment with that 3 mm screw that I referred to. Glenn


Glenn, Since the reviews are a little unfavorable at Jaytan in regard to the cord refunds I think I'll do what you did, have mine rebuilt. So was the rebuild $213. than ? Do I still have to pay $50. for a test ? Well after complaining to the BBB and writting bad reviews at many forums I finally after 1 year and 3 months got my core refund from Euro Port Parts on the rebuilt wur LOL. Gene

Gene, yes, that's pretty much why I sent mine, to avoid the potential core hassle. Also, I read on one of the boards where a Pelican rep stated that they had used Jaytan for some of their business and had no particular issues (I personally have high regard for Pelicn as a business). The $50 testing was rolled into the $213. If it would have tested okay, I would have owed the $50. When they got mine rebuilt, they called for my credit card #. It was $212.99, including shipping. No shipping on the second one either. Glenn


This is to inform anyone who's tempted to use Jaytan Industries Inc. "Stay away from these guys !! " They kept my money ( $213.00) and never sent me my fuel distributor to me so now looking for another FD to get rebuilt. Also don't do what I did, use a debit card use a credit card as you will have some protection from fraud. Gene

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