Thread: Persistant misfire at idle, stalls at idle in gear

Originally Posted by Swarvey

The Vehicle: 1987 Range Rover Classic, 3.5 Injected V8
* Yesterday I found running vac advance at manifold pressure, instead of throttle body pressure smoothed it all out as in totally gone, problem solved, tried more advance by timing light instead so I could put the vac lines on properly, didn't have same effect,

it drove and idled perfectly in gear, at stop lights without stalling,

replaced all ignition wiring (pickup to amp, amp to coil, ign to coil) with new 3mm Twin-Sheath, soldered and heatshrunk crimps and gapped the pickup coil and stator today and things just got worse
.

G`day ,

using manifold vacuum as you did moves the backing plate to its full travel which inturn changed the orientation of the wires within the distributor .

From what you`ve written it seems you have changed these wires but from your explaination it`s saying the fault is with these wires or related .

The fuel pressure regulator requires vacuum at low revs/idle to lower the pressure so the pipe not being connected should affect it at low revs/idle

Not sure if I'd add anything to the white/black wire, there may be a resistor between the ECU and the coil. Take the tach wire straight to the coil (-). If your coil is producing random firing from interference your tach will read high.

Setting the mixture by adjusting the MAF is pointless unless your fuel pressure regulator is working correctly. Try a pressure gauge. Vac hose off means increased fuel pressure at idle so no good once under load. That is why I do a "dynamic" tune, i.e adjust the MAF and see how it drives. Don't need a fancy analyser to feel it go better.

@PLR - I found that it actually required very little suck to even out the rpm at idle, I was just doing the basic test (suck on pipe and see if it holds your tongue) to see if the vacuum advance was working.. I only tapped into the manifold vacuum because it was constant. Constant vac advance smoothed out the idle so I left it there n took it for a drive.

When I say I changed the wires, I didn't mean on the pickup coil itself, there's the old crimps from the OEM amplifier, so I made new cables to go from here (via small blade crimps) to the Bosch module, as per bee uteys guide. I'm guessing you're thinking something along the lines of broken wires on the pickup coil? Put the dmm on the crimps and managed approx 3.1k ohms, which is okay according to rave. Sucked on vac to see if resistance changed and it was stable.

@bee utey - yeah I'll remove the tach from there too, realistically I don't need it, and it doesn't agree with ( has at least once shown higher rpm than) my new dmm with inductive pickup.

I totally agree, I've tuned it "dynamically" many times, but when troubleshooting I like to see the numbers and compare them to factory spec too aid in diagnosing the engine. When I get serious about tuning I've got more hardware at my disposal, o2 sensors, afr gauges among other things. Saving for a DIY megasquirt too.