treetotreef Posted January 21, 2010 Report Share Posted January 21, 2010 I've decided to start tracking down a misfire that has been in my '98 S8 since I bought it over a year ago. I’ve been slow to open the toolbox on this one because the engine runs perfect on LPG, but not on Petrol, and as LPG is half the price, I was never really that bothered. The story goes like this… I can start the car every morning, it runs on petrol up to 50˚c water temp, then switches over to gas. It will then run all day and night on gas until the gas runs out. If the gas runs out, or you want to want to switch to petrol, it will misfire. Mostly on cylinder 2 but it could also misfire on 7, 3, 5 etc. Not the end of the world, especially as LPG is half the price of petrol and 95% of the time I run on it. But it is annoying if you do a short run, and the LPG ECU won’t switch over to gas for what seems to be a preset time period (~3 minutes or so). I had also come up with the theory that when running on LPG, the petrol fuel map was being trimmed from the lambda readings, but the ECU doesn’t know what fuel its running on, so it trims the petrol map accordingly. Then when you switch back to petrol, the map was trimmed way outside a workable range, and hey presto…misfire. But VAG-COM showed up sensible trims on part throttle (motronic 5.4 does not seem to adjust fuelling on pull throttle), so that theory was ruled out. But it’s bothering me now, so time to find it! I've ruled out the ignition side of things for 2 reasons 1. It runs well on gas and 2. I've tested the coils and coil amplifiers for resistance, with good results A quick compression test rules out burnt valves and worn bores etc, so this just leaves the fuel system. I checked the resistance of the injectors a while back but the engine was cold then and they showed up no problems…and so the problem went on the back burner. This time however I measured them just after the misfire occurred. This revealed that 3 injectors were high resistance and one had a completely open circuit. I’ve seen heat soak to upset crank and cam sensors before but never injectors. It’s a pretty well sealed environment under there and any heat created must struggle to get out. A set of brand new S8 40v (engine code: AVP) injectors cropped up at an ok price on eBay. Audi Part number: 077133551Q Bosch Part number: 0-280-155-988 These have are similar high impedance to the S8 32v (engine code: AHC) injector… Audi Part number: 077133551L Bosch Part number: 0-280-150-463 …with an identical flow rate. The only thing I was missing was a length, but I took a chance anyway on them. Unfortunately they are about 12mm too short, and unless I machine up some cups to make up the space or hack up the rail to let it sit deeper into the engine, they won’t fit. Some more injector info here: AUDI Part no. Engine Code BOSCH Part no. LBS/HR CC/MIN GRAMS Resistance PSI BAR 077133551L AHC 0-280-150-463 18.25 191.8 138 15.9 43.5 3 077133551Q AVP 0-280-155-988 18.25 191.8 138 14.5 43.5 3 I then found a complete 32v AHC set with rail for a good price from: VRS Home, A quick multimeter check confirmed that these were all the correct resistance (15.9 Ω) fitted 3 and now she’s back to full working order. For me, its not worth putting them on an ANSU machine to check flow rate and spray pattern. I don't do enough mileage to justify the cost of that for 8 injectors. One extra benifit i do notice is thats its now seamless on changover between petrol and LPG and back again. before it would drop ~300rpm and if under load, the rpm drop (~500rpm) would be registered as a problem by the prins VSI system, and it would revert back to petrol. a nice bonus. Link to comment Share on other sites More sharing options...
Figure11 Posted January 21, 2010 Report Share Posted January 21, 2010 Your not going to like this but I have just shell out the best part of £500 on LPG parts to cure a very similar set of symptoms. I would start on petrol from cold and suffer horrendous misfire on one bank.. Soon as I hit 50 deg C the LPG kicks in and the car runs fine. Switch back to petrol and back to horrendous misfires. It turned out to be the ECU clones used by my Prins system. One had gone duff and was fooling the cars ECU into thinking the petrol injectors were firing when they weren't. Ok on LPG because the LPG injectors were being fed with gas and the engine runs fine. Not ok on petrol because the petrol injectors aren't firing the LPG injectors were. But with no LPG being fed because the system is switched to petrol and no petrol because the petrol injectors aren't firing so the whole bank is registering misfires as the LAMBAs are off the scale etc etc. The cure was a new ECU Clone. The cost was the fact that Prins changed the design of the ECU clones from hard wired to plug in after my system was fitted. So 2 new plug in ECU clones and the associated wiring and software update and 5 hours of labour to fit. Only good news if it happens again I can change them myself. HtH Link to comment Share on other sites More sharing options...
treetotreef Posted January 21, 2010 Author Report Share Posted January 21, 2010 Apologies, that list in the middle was meant to come out as a table of info. does anyone know how to edit a post? can't find the icon or link anywhere! Figure 11, when you say ECU clones, are they internal in the ECU? Link to comment Share on other sites More sharing options...
Figure11 Posted January 21, 2010 Report Share Posted January 21, 2010 You can only edit for a fixed period after the original post. On the Prins system there are 2 ECU clones, one for each bank of a V8 and the LPG ECU. The job of the ECU clones is to intercept the firing pulse to the Petrol injectors and tell the Cars ECU the the petrol injector has fired. The pulse is redirected to the LPG ECU where it is modified and sent to the LPG injectors. The Cars ECU thinks all is normal as the clones are telling it that is the cas. In my case one ECU clone was malfunctioning and telling the cars ECU all was ok when it wasn't. The ECU clones are cigarette packet size and black in colour. Link to comment Share on other sites More sharing options...
treetotreef Posted January 21, 2010 Author Report Share Posted January 21, 2010 got you. I've seen those in mine,stacked 2 high. useful to know. Have you got the software for the Prins ECU? or more importantly, the connection cable. I have a copy of the software, but not sure if its worth the £40 for the cable off ebay... Link to comment Share on other sites More sharing options...
Figure11 Posted January 21, 2010 Report Share Posted January 21, 2010 I have no software or cable. have a look at the ECU clones if they are plug in then you are laughing. (if that is what is wrong) if they are hard wired then Link to comment Share on other sites More sharing options...
treetotreef Posted January 21, 2010 Author Report Share Posted January 21, 2010 From memory they're hard wired. hopefully they won't fall over in the near future, but I'm glad that i found my 3 dodgy injectors and my problem is finally solved now. its been annoying me for too long! Link to comment Share on other sites More sharing options...
treetotreef Posted January 22, 2010 Author Report Share Posted January 22, 2010 Better internet connection at work so I'll upload some pics and more info Here's a good website for bosch injector data... http://www.usrallyteam.com/content/products/injector/Bosch_Injector_data.xls just press Ctrl + F and type in your injector number Also attached are full info for the 32v and the 40v injector in the zip file Audi S8 Bosch_Injector_data.zip Link to comment Share on other sites More sharing options...
Tonsko Posted January 22, 2010 Report Share Posted January 22, 2010 Sorry to go a bit OT, but do you notice a drop in power running the LPG or is it just the same? Link to comment Share on other sites More sharing options...
treetotreef Posted January 22, 2010 Author Report Share Posted January 22, 2010 Before, with unknown poor performing injectors, I'd have said no. But since sorting out the dodgy injectors, i'd say on petrol is a fraction more responsive...but loosing a few horses on something that already has 340 of them already is a small price to pay for running on fuel thats half the price of petrol. The other thing you have to ask yourself is how long do you really spend at full throttle? Link to comment Share on other sites More sharing options...
Figure11 Posted January 22, 2010 Report Share Posted January 22, 2010 Exactly wot he said Link to comment Share on other sites More sharing options...
Tonsko Posted January 22, 2010 Report Share Posted January 22, 2010 So the review of LPG given elsewhere on this site that says it leads to long term reliabilty problems, have you noticed anything like this? And how'd'you reckon it would go with my .:R32? Link to comment Share on other sites More sharing options...
Figure11 Posted January 22, 2010 Report Share Posted January 22, 2010 I have had absolutely no mechanical or electrical problems with the car due to the LPG. One LPG component has failed, but then so do petrol components. If you do high mileage then I would not hesitate in advocating a Prins system. Link to comment Share on other sites More sharing options...
treetotreef Posted January 23, 2010 Author Report Share Posted January 23, 2010 Likewise I'm a huge fan of LPG. if your doing the mileage, its worth fitting. If your not doing the mileage, its still worth buying a car with it already fitted. I've had no problems whatsoever from my Prins system either. I bought the car with the system fitted. it was fitted in '02 at 55k, now has 165k up on it. Link to comment Share on other sites More sharing options...
Tonsko Posted January 23, 2010 Report Share Posted January 23, 2010 Hm. Cheers for the feedback. 'Mileage' - are you talking kind of over 12K? Link to comment Share on other sites More sharing options...
Figure11 Posted January 23, 2010 Report Share Posted January 23, 2010 I took me 21k to recoup the cost. For me that was 11 months. Link to comment Share on other sites More sharing options...
ainarssems Posted April 29, 2010 Report Share Posted April 29, 2010 I tried to send a private message but have not received reply, I am new to tyresmoke forum so maybe did something wrong. Anyway I am looking for S8 AHC fuel injector and was wondering if treetotreef have any left over for sale. Or in fact if anybody at all have some for sale. Link to comment Share on other sites More sharing options...
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