Whew I've been VERY busy with the 245 lately. After having done a cylinder head overhaul about a month back (valve job, shims, seals) adding the IPD VX camshaft (set to -2.5deg currently), I decided to attack my braking system this week. So I got a new master cyl, front lines/hoses, rebuilt front calipers, new pads/rotors installed and cleaned and re-packed my front wheel bearings and changed the wheel seals. So I did all this work and now I've got a funny "misfire" condition. I first noticed it after I finished the cylinder head work a month ago. The check eng light would just barely flash on decelleration just before idle and this would only happen every once in a while. I attributed this to the new cam and didn't think anything of it.

Now I get a very consistent misfire when the car is up to temp. I would characterize it as a "load fault." It can happen at a wide range of RPMs and the car can be in gear/ out of gear, driving/ static. Under acceleration it doesn't seem to occur, but when I reach a speed under 55mph and hold it or slightly decellerate I will get a misfire. When reaching and holding at 40mph, the misfire will occur repeatedly about every second. When this happens, the check eng lite flashes during the misfire and then goes out. No codes stored on the ECU during all this.

So I've tried disconnecting several things to try to isolate this (see list below). The car runs best when I totally disconnect the AMM/MAF but I can't be sure if this is simply masking the problem by using a fixed fuel/ign map or if I can certainly say something is wrong in the AMM circuit. With the AMM disconnected it will still run a bit rough at times, but I can't be sure if that's still my problem shining through a rich mixture. Swapping AMMs and ECUs has no effect on this problem - same fault! I'd like to make up a test harness to run back directly to the ECU for the AMM... has anyone ever done that? I'm tending to suspect the harness or the AMM connector at this point. Could it be ignition related? So what's the best way to functionally test the AMM circuit? The Bentley manual on page 241-15 gives procedures but says, "terminal 21 grounded." Should I use a jumper to ground out terminal 21 on the ECU connector with the ignition on? Also, I wish I knew more about using an oscilloscope. I had the basics on using these about 15yrs ago, but never on a car so I don't really have a foot to stand on. Anyone know of a good O-scope for auto diagnosis? I like the PC-based scopes and the VELLEMAN-PCSGU250 interests me. I kind-of understand how these are used to analyze injector volts/current/waveform and secondary ignition strength, but can an o-scope be used to diagnose the AMM circuit? How?


PARTS SWAPPED NOW TO INVESTIGATE PROBLEM:
- ECU swapped with my spare - same problem
- AMM swapped with my spare - same problem
- ignition module w/ spare (inner L fender) - same problem
- intake air hose - same problem
- knock sensor - same problem
- main relay/fuel pump relay - same problem

TESTED SO FAR:
- O2 sensor: using analog meter stays within normal operating range. I noticed that when the misfire occurs, the sensor V drops to near zero V (does not seem to precede the misfire but seems to be a result of the misfire).
- fuel pressure: about 38psi at idle, 42-45psi when removing press reg vacuum line.
- vacuum is good - about 15in at idle, pulls to 25in at decel. When the misfire occurs, I get a drop in vacuum (ie. toward 0in).
- disconnect TPS sensor - same problem occurs
- disconnect O2 sensor - same problem occurs
- disconnect idle air motor - same problem
- make/break connection at: crank sensor, AMM, O2, knock, ign coil module, ign coil primary, spark plug leads, ECU main connector

PARTS CHANGED DURING CYL HEAD WORK (1 month ago):
- IPD VX camshaft & sprocket (now set to -2.5deg)
- new timing belt, tensioner
- new Bougicord crank angle sensor
- new Bosch knock sensor
- NGK plugs (Bougicord ignition wires are fairly new too)
- new cap/rotor
- fuel injectors sent to RC engineering for rebuild/clean/balancing
- new Bosch fuel press regulator


thanks guys!
Steve
1990 volvo 245, LH 2.4