WNX700H Posted April 20, 2019 Share Posted April 20, 2019 My 88 Targa has become difficult to start when hot after it has stood . Eventually starts but then runs rough for a short time with fluctuating idle speed eventually sorts itself out and then ok . Had similar issue when starting from cold this morning. Most likely causes please and areas to investigate? Link to comment Share on other sites More sharing options...
WNX700H Posted April 21, 2019 Author Share Posted April 21, 2019 Ok so same problem when cold now also ...... engine cranks over but will not fire initially when it does, it is not firing properly but once it does start to fire it will eventually sort itself out and idle normally then everything is OK. Link to comment Share on other sites More sharing options...
NigelJ Posted April 21, 2019 Share Posted April 21, 2019 The first thing to check out is the DME relay under the passenger seat. They age and the solder joints can crack and cause your symptoms. They can be repaired but its better to fit a new one from Type911. Most folk always carry a spare. Link to comment Share on other sites More sharing options...
ALEX P Posted April 21, 2019 Share Posted April 21, 2019 Yep, could be DME relay. Probably worth whipping your idle control valve off and giving it a clean, it's only a quick job. Could be your crank sensor messing around too. Not a difficult job but a bit fiddly. Link to comment Share on other sites More sharing options...
Chris T Posted April 21, 2019 Share Posted April 21, 2019 Sounds like it could be the crank sensor. I had the exact same issue recently. One sensor reads the teeth on the starter ring, the other reads TDC. If the ECU sees no TDC reference it refuses to issue fuel = no starty. Matt does a reasonably priced version, or like me, use the BMW listed one. Bosch no. 0261210002 Chris. Link to comment Share on other sites More sharing options...
Magnum Posted April 21, 2019 Share Posted April 21, 2019 Hi Chris Cn I ask a potentially silly question, but rather double check than feel even sillier later......both sensors (ie crank sensor reading teeth and the TDC sensor) are exactly the same part - ie Bosch 0261210002? Regards Link to comment Share on other sites More sharing options...
Chris T Posted April 21, 2019 Share Posted April 21, 2019 Yes, both the same. Link to comment Share on other sites More sharing options...
WNX700H Posted April 21, 2019 Author Share Posted April 21, 2019 Swapped DME relay yesterday no change. If its the crank sensor then interested to understand how the car eventually starts if the sensor is no longer functioning? Link to comment Share on other sites More sharing options...
SilverWT Posted April 21, 2019 Share Posted April 21, 2019 (edited) 22 hours ago, WNX700H said: Eventually starts but then runs rough for a short time with fluctuating idle speed eventually sorts itself out and then ok If this an accurate description of the symptoms it sounds more like the cylinder head temp sensor playing up than anything else. The other sensors and the DME relay either work or don't work, even if it is an intermittent fault that is sometimes there, sometimes not, and maybe heat dependant. With a dodgy CHT sensor it is difficult to start and idle is effected until up to full operating temp. Mark Edited April 21, 2019 by SilverWT Link to comment Share on other sites More sharing options...
WNX700H Posted April 21, 2019 Author Share Posted April 21, 2019 Description is accurate but the time it takes to run smoothly is seconds as it clears itself so don't think temp is a factor unless the temp sensor gives no indication of temperature at all Link to comment Share on other sites More sharing options...
SilverWT Posted April 21, 2019 Share Posted April 21, 2019 24 minutes ago, WNX700H said: unless the temp sensor gives no indication of temperature at all Which can be the failure mode. At 15 - 30C the resistance should be 1.4 to 3.6 k ohm across the terminal in the white plug at the top of the bracket on the side of the nearside inlet manifold towards the front of the car. High resistance = rich mixture, low resistance = weak mixture. Mark Link to comment Share on other sites More sharing options...
GrahamTompkins Posted April 21, 2019 Share Posted April 21, 2019 I’m with Mark on this...similar symptoms on our Honda racecar yesterday..swapped the CHT ..instant fix..much relief as my son’s racing tomorrow. Link to comment Share on other sites More sharing options...
Chris T Posted April 21, 2019 Share Posted April 21, 2019 If it IS the CHT sensor, like the BMW crank sensor, is there a cheaper source? Crazy money these days for such a simple thing as I found when I did mine recently. I did look at Ebay £12 versions but nobody could tell me resistance values. They tend to be a ring fitting but you could use the old one as a bolt to attach the new. Chris Link to comment Share on other sites More sharing options...
angusb Posted April 21, 2019 Share Posted April 21, 2019 To my knowledge the CHT sensor is unique to 911`s , so replacement unfortunately requires Porsche Tax. The body of the sensor is a heavily recessed into the cylinder head, although the resistor element is likely to be pennies, there is no real way to repair. New sensor are also wired slightly differently, having improved twin earths. Link to comment Share on other sites More sharing options...
Chris_911 Posted April 21, 2019 Share Posted April 21, 2019 I agree with Angus - not aware of any alternative to the Porsche OE sensor. Unfortunately Link to comment Share on other sites More sharing options...
Type911 Posted April 21, 2019 Share Posted April 21, 2019 (edited) We have the NLA Bosch sensor in stock about £50 saving Edited April 21, 2019 by Type911 Link to comment Share on other sites More sharing options...
NigelJ Posted April 21, 2019 Share Posted April 21, 2019 Another suspect is the fuel pump which can go flakey. Can you hear it when first switching on the ignition? There is also a procedure for checking the fuel pressure, which I have forgotten, but which will be on here. Try a search. Link to comment Share on other sites More sharing options...
angusb Posted April 21, 2019 Share Posted April 21, 2019 Unlikely to be fuel pump, when they go the car would not restart. Link to comment Share on other sites More sharing options...
proporsche Posted April 22, 2019 Share Posted April 22, 2019 do you have any aftermarket alarm in your car?? Ivan Link to comment Share on other sites More sharing options...
WNX700H Posted April 22, 2019 Author Share Posted April 22, 2019 The car will start albeit reluctantly and this is a recent change of behaviour which suggests something is not as it should be. No alarm fitted. Link to comment Share on other sites More sharing options...
angusb Posted April 23, 2019 Share Posted April 23, 2019 On 4/22/2019 at 3:45 PM, WNX700H said: No alarm fitted. With all the toe rags about, unfortunately one is a necessity. Re read your original post, when attempt to start, is the engine turning over at normal speed? Link to comment Share on other sites More sharing options...
proporsche Posted April 23, 2019 Share Posted April 23, 2019 no alarm is necessary if it is wrongly installed;-) all you need is my 911-Lockshield™ Link to comment Share on other sites More sharing options...
proporsche Posted April 23, 2019 Share Posted April 23, 2019 as ungus said temperature head sensor is very common problem.To find out what do you have ,disconnect your white plug and look at the male connector.If you see one pin inside you have the very old type of sensor and it should be replaced ...these two pins inside the white plug Ivan Link to comment Share on other sites More sharing options...
WNX700H Posted April 24, 2019 Author Share Posted April 24, 2019 Car cranks over fine , cylinder head temp sensor is single wire, have ordered a new one. Guess I only need to plug in the new sensor when I get it when engine is cold and see if it starts as OK. Then cause is proven.... ok I then need to fit it as I realise very quickly the engine will not run properly as mixture will not adjust if I am just sensing ambient temp.... but ok approach I think to prove the issue Link to comment Share on other sites More sharing options...
angusb Posted April 24, 2019 Share Posted April 24, 2019 If you have not already got the workshop manuals, Nige has kindly downloaded them,see under "All other technical issues" section. When you come to fit the new sensor, you probably need a homemade extra long 10mm socket, with a slit up the middle to allow for the wire.Some have managed with pair of needle nose pliers. The rubber grommets on the wire go through engine tinware, some soap or Silicon grease spray makes them mucho easier to fit. Link to comment Share on other sites More sharing options...
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