'94 R129 SL500 Injectors

[E55BOF]

Still plugging (no pun intended) on with the thing - still misfiring on no.5 cylinder, which is the one it was misfiring on originally. New distributor caps & rotors, new HT leads, new plugs, have made no difference. Some will recall that when I changed the plugs, the one in no.5 showed no signs of combustion having taken place recently.

I took a look at the throttle body wiring loom's internals, and it's OK; a bit rigid and crispy, but not in need of immediate replacement, so I'm looking to get the car running properly before I replace it.

It now looks 95% certain that it's the injector. I've done over 50 miles since fitting the new plugs, but the brand-new plug in no. 5 is as shiny as when I fitted it. Just in case, I refitted one of the old (and in pretty good nick) plugs, but the misfire is still there. The injector for that cylinder has the same resistance (c. 14.5 ohms) as two others, so it seems highly likely that either it is blocked, or that the refurbished loom from SiLec, either through being fitted by a thick-fingered numpty (not me...) or just possibly because it is faulty, is not activating that injector. I've cleaned the contacts at the injector; no change.

Sunday looks like a good day to get my hands dirty again. It seems to me that the easy way to find out without special test kit (which I don't have) is to swap the questionable injector to another cylinder, and see if the misfire moves with it. It looks quite straightforward, if time-consuming, to remove the fuel rail and injectors, but I can't tell from visual inspection how the injectors are held into the fuel rail. Could some kind soul enlighten me?

I've also read somewhere that some injectors are fitted with an internal filter; does anyone know whether that's the case on a 1994 SL500?

The car is going in to WGMB on Monday morning anyway, so if I can't identify and (with luck) rectify the cause of the misfire that will still have to be done, and I'll get the codes read and the gearbox oil and filter changed anyway, though the gearbox is greatly improved with being driven.

I'm still getting there...

 

[jonnyboy]

Dennis, have you done a compression test? Please tell me you have???

 

[Terry_]

Dennis your car should have the LE fuel injection.
It's ultra rare for the injectors to fail

 

[E55BOF]

Compression test Sunday morning; didn't see the need to mention it. I'll be surprised if it's that, though; for it to be so low as to give no combustion whatever, I'd have thought you'd need either an exhaust valve stuck open, which would presumably contact the piston, or a hole in the piston. Either way, I'd expect lots of smoke in the exhaust, and there is none. I'll know on Sunday.

Thanks for that, Terry; from the impedance reading, the injector is OK, but it might be blocked. I'll know on Sunday, if somebody can tell me how the injectors are secured into the fuel rail...

 

[E55BOF]

Compression 180 psi; I knew it wasn't fuelling in that cylinder, so just set up the gauge, then started it (cold) and let it idle for a couple of seconds. I had a bad moment at first when it looked like 125 psi, but that was actually 12.5 bar. Phew!

Too cold and snowy to muck about much, but running it with the multimeter on the two connections of the loom connector gave a steady 13.3 volts, as did the same test on another injector (for a cylinder that is firing). I surmise that no fluctuation (when the ECU sent its signal) was observed because the voltage change was of such short duration that the multimeter did not have time to react before normal voltage was restored. Marvellous stuff, that there electrickery...

The cylinder compression is good; the plug is sparking; there's no combustion taking place. The problem has to be fuel, and the tests I have done indicate that the coil in the injector is OK, and the loom from the ECU is serviceable. Unless the ECU signal to that one injector is corrupted, or somehow sending its signal at the wrong time, unless there's a mechanical fault with the injector I can't see how it can be anything other than a blockage.

I'm not sure how much info the OBD setup on a car that old will provide, but it goes in tomorrow, and we'll see what STAR says. More anon...