MikeAK Posted July 17, 2009 Share Posted July 17, 2009 Hi, Been looking today at a mates 1992 RR Classic 3.9 which has a bad misfire. Symtoms are: Starts and idles OK but if throttle snapped open it pops back through the air filter. Drives O.K. on small throttle openings and low speeds but on wide throttle openings or moderate throttle opening at speed it pops back through air filter. This started a few weeks ago and has got gradually worse to the point where my mates stopped driving it. Things tried so far: Changed plugs for new NGKs - no change. Swapped plug leads for the ones of my RR - no change. Swapped Dizzy including rotor arm and cap for the ones of my RR - no change. Swapped coil for the one of my RR - no change. Spark checked on main lead and its good. Timing set at 6 BTDC using strobe vacuum disconnected. Swapped ECU for the one of my RR - no change. Swapped AFM for the one of my RR - no change. Checked all signals e.g AFM, TPS, CLT at the ECU connector by probing with ECU connected- all O.K. Fitted a Non-Cat tune resistor to take Lambda sensors out of the equation - no change. Non of the plugs look any more sooty than the others. Old plugs looked like it was runing OK but with "surface" soot from cold starting Ran the pump by linking the relay contacs and fuel pressure is 35.5psi and holds up if the injectors are "blipped" by earthing at ECU connector. Compression test gives 150 psi plus minus 8psi all cylinders Can't think of anything else to try around ingnition and fuel, so am now wondering if its somthing for fundamental in the engne like one or more worn cam exhaust lobes. What makes me think this is I've had this symptom on a cavalier and a sierra in the past both of which turned out to be a worn exhaust cam lobe. I think what happens is because the exhaust valve is barely opening the large amount of hot/burning gas at large throttle openings cant get away and comes out the intake valve at the next induction, hence the poping back through the air flter. Worn exhaust lobe doesnt affect compression test. But before we pull the inlet manifold off to have a look at the camshaft and followers does anyone have any other ideas around fuel and ignition or anything else? Thanks Mike Quote Link to comment Share on other sites More sharing options...
geoffbeaumont Posted July 17, 2009 Share Posted July 17, 2009 Did you swap the ignition components one at a time or fit all the parts from your car simultaneously? Could be more than one faulty component. Only other electric thing I can think of is a poor earth, though I've not seen it cause anything that dramatic. My money's on a worn cam lobe, mostly likely the one at the back. Quote Link to comment Share on other sites More sharing options...
FridgeFreezer Posted July 17, 2009 Share Posted July 17, 2009 Sounds more engine management related - ignition amp would be first thought if you haven't already swapped it. Quote Link to comment Share on other sites More sharing options...
SteveRK Posted July 17, 2009 Share Posted July 17, 2009 Sounds like a classic case of fuel starvation? Fuel pressure regulator faulty - bypassing too much fuel? Fuel filter clogged fuel pump filter clogged Fuel pump 'weak' It appears that the fuel system is able to provide enough fuel for moderate consumption. Quote Link to comment Share on other sites More sharing options...
MikeAK Posted July 18, 2009 Author Share Posted July 18, 2009 Hi again, We did swap the components one at a time. Comes from my engineering training of only change one thing at once so you can quantify whats happening. Will try changing the whole lot at once on Sunday. The ignition amp is on the side of the dizzy so was swapped with the dizzy (had to make up some leads, different connectrs). To be honest from past experience I thought that would be the problem but not this time. Fuel pressure seems to hold up when manually blipping the injectors. Held them open for about a second which is a fair old volume of fuel at once. Obviously dont want to do to much of that or we'll fill the sump with fuel! But will try changing the fuel filter. I tend to agree with geoffbeaumont that its the camshaft but will investigate further first. Should it come to it I have seen Island 4x4 advertising camshaft kit (camshaft, followers, chain, gears, gaskits etc) for £110. Is this a reasonable price and does anyone have experience of the quality. Thanks again Mike Quote Link to comment Share on other sites More sharing options...
steve_d Posted July 18, 2009 Share Posted July 18, 2009 Rather than pull the inlet I would start by putting a DTI on each of the valves to see how much lift each is getting. From my experience of a worn lobe the symptoms you describe are pretty late on into the process so you may be able to spot the problem by eye or just by holding a steel rule beside each spring. Steve Quote Link to comment Share on other sites More sharing options...
HoggyN Posted July 19, 2009 Share Posted July 19, 2009 Air leaks in the induction system? I'd check that before the cam, I had an V8 SD1 with virtually no bumps left on the cam and it ran fine. Quote Link to comment Share on other sites More sharing options...
MikeAK Posted July 19, 2009 Author Share Posted July 19, 2009 Hi, Will check the check for intake leaks, will have to pull the plenum off to get the rocker covers off anyway to check valve lift as suggested by steve_d or to pull the inlet manifold. BUT not for a couple of weeks. I'm off to sunny Turkey for 2 weeks in about 8hrs so sort of lost interest for now! Will post back with whatever the problem is when I get back. I'm sure it won't have healed up by itself while I'm away! Thanks for the help so far Mike Quote Link to comment Share on other sites More sharing options...
FITZ Posted July 20, 2009 Share Posted July 20, 2009 Ignition amplifier? Unless it was on the dizzy you tried. Quote Link to comment Share on other sites More sharing options...
MikeAK Posted September 22, 2009 Author Share Posted September 22, 2009 Hi, Its a while since I posted about this because I went on hols then my mate (whose RRC it is) was working away for a couple of months. Anyway he's back now and we've had another look over the weeknd. Swapped all the ignition components at the same time as suggested by geoffbeaumont in case more than one item faulty, this did ot cure it. So we next we took steve_d advice and pulled the rocker covers to check valve lift. It was imediatley clear by eye that No7 exhaust had considerably less lift than the others. So we dismantled further and the camshaft looks like this. Also all the followers are very dished on the bottom. So parts on order and hopefully it will be cured when its all back together. We're also now thinking that as its stripped down this far should we pull the heads while we're at it to decoke and grind the valves etc. Regards Mike Quote Link to comment Share on other sites More sharing options...
roverar8 Posted November 26, 2013 Share Posted November 26, 2013 have the problem fixed after replacing the cam? Quote Link to comment Share on other sites More sharing options...
Fanakapan Posted November 27, 2013 Share Posted November 27, 2013 have the problem fixed after replacing the cam? Thats good As far as ignition amplifiers go, I'd advise everybody to do the remote mounting mod, but make your own bracket so the amp can be swapped out in minutes, it saves hours of checking other stuff Quote Link to comment Share on other sites More sharing options...
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