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

Well thank you guys for all of your help! Great idea about the IR temp gauge, BTW. I do have one but I didn't really give it too much thought until you mentioned it here - I didn't try it and I don't know if the cheapo one I have will be quick enough or sensitive enough to show anything, but it'll be a good idea for the future to help isolate cylinders.

After spending the whole day doing all the tests in the Bentley manual and a few tests of my own after studying the wiring diagrams closely (lots of mis-labeled stuff so it was tough to find components on the 4page diagrams without lots of flipping around), I finally found the problem. Everything checked out just perfect and all functioning correctly! It turned out that I had the WRONG SPARK PLUGS in the thing! I don't know where I ordered them from or if they came from the auto parts store, but I ended up with NGK BP6ES instead of the correct BPR6ES. I'm guessing that these were non-resistor plugs. About a month ago I bought a cheap $6 spark plug tester light at Harbor Freight that plugs inline with one ignition wire at a time. I tested each cylinder and found that each plug continued to fire during the misfire condition EXCEPT the #2 plug... so I changed the plug. It still misfired on #2... so I changed the #2 wire using my old Bosch leads. Still misfired on #2... so I changed the dist cap & rotor... on and on until I swapped in my old coil. It was progressively getting better, but still mostly there. That's when I looked at the plug I pulled and found the wrong number. So I changed all the plugs and went back to my old Bosch ignition leads and now there's no misfire.

Now I have yet to drive it, but I'm pretty sure it was the plugs. Pretty interesting how it manifested only and consistently in the #2 cyl on a light load. It makes me wonder and theorize that under increased cyl pressure I was getting more resistance. But why in only #2? Some kind of goofy counter-EMF magic due to the length of that ignition lead????

I did sort-of devise my own functional check of the AMM during operation. I de-pinned the #2 wire of the connector at the AMM and connected an ammeter in series with it. The #2 (green/yellow) wire is the ground path back to the ECU for the AMM platinum wire so I connected the positive of the ammeter to the AMM pin and the neg of the meter to the green/yellow wire. In operation I was reading 0.10 milliamp at idle. Revving the engine up gradually you could see current increase up to 0.14 milliamps and then back off to about 0.12 to hold the higher rpm. The ECU uses this change in current to make its fueling decisions. If I had a more reliable way to keep that connection, I would have been able to watch it while driving.

So I still have to check my valve clearance and experiment with my new VX cam timing... and then, of course, test drive to verify that this problem is solved for sure.

Thank you all for your ideas and help!
Steve

I was able to answer my own question about pin #21 at the ECU. Looking over the wiring diagram it seems that by grounding pin #21 (with ignition on and connector still plugged into the ECU) you are signaling the main/fuel pump relay to keep the main ECU contact closed to apply power to the AMM even though it would not be powered because there is no RPM signal. Although when I removed the grounding connection, I didn't hear the relay click off... hmmm.

This was the test on page 241-15 of the Bentley manual for the AMM power supply

Were your new plugs buy and chance Platinum.

Ron J

Since you have had the head off I'm going to assume that the exhaust manifold shield can be easily removed. Get the engine to operating temp and induce the miss. Then use an infrared temp probe to check exhaust temp at each exhaust port on the manifold. A misfiring cylinder will stand out by having slightly lower temp than the others. Unfortunately this won't work if you have a random/multiple misfire [DTC P0300], but it is a quick way to diagnose misfires. I have a pricey name brand but I have found that Sears has a decent one. Make sure as well that your new injector seals are properly fitted and tight.

You've replaced a lot of parts that would have nothing to do with a misfire.
Can you tell whether the miss is on one cylinder consistently, or if its a random misfire among all cylinders?
Have you pinched or plugged the various hose connections to the intake manifold, breather and power brakes to see if that changes anything?
Because you had the valves ground, and because the miss happens when fully warm according to your description, have you checked the valve clearances hot?
Have you pulled the injector connectors with the engine running?
Have you checked that the O2 sensor is switching about one volt and at a good rate? Disconnecting it won't help anything.
Did you pull plug wires with the engine idling to identify a cylinder, if it's not a random miss?

have you verified your timing is correctly set?
one tooth off perhaps.

I've not checked timing yet, but I did loosen the t-belt tensioner to re-tension the new belt. This didn't help resolve the problem or change the characteristics in any way. I will be re-doing the timing so that I can put on my stock cam sprocket to set the new VX cam to zero. Interestingly, I changed the rear plastic timing belt cover (I think it was either Uro or Scan-Tech brand... ugh!) and it came with no alignment marks for the cam or the layshaft. I did my best to scribe them in by comparing to the old rear cover, but who can say for sure if I got it right? I did fortunately use a nice new Continental T-belt which has the nice white stripes and I did verify that everything was set properly. Well, we'll see what the timing is after I put on the orig cam sprocket.

And I do agree that changing a whole hoard of parts introduces the unknown to the situation. I'm effectively testing out all these new parts now! I did pay the extra premium to get the closest to OEM on everything (Bosch and Bougicord etc.) but I realize everything has slipped in quality. The only one I'm worried about is the crank angle sensor since it's a pain to get at with the cyl head on, but the misfire happens at a light load, off-idle so I think a bad crank angle sensor is a long-shot.

Dan, all the spares that I swapped in were known good parts that I had on the car previously. On the AMM, I've ordered another used one from ebay. I've cleaned the harness contacts on both of these and I've used MAF cleaner spray to clean the element - same problem persists. No known air leaks between the AMM and throttle plate. I swapped in a new main air tube/hose. The intake and throttle body were "anti-septically" cleaned during my cyl head job. Disconnecting the idle air motor had no effect on the problem. I did remove the throttle switch (pseudo-TPS) when I cleaned the throttle body. It makes me wonder if there is an air leak around the throttle shaft? I've got another throttle body I can try. No OBD codes have been stored.

thanks!
Steve

Use Art's method of counting teeth and lining up the dizzy mark then you know the cam is correct.

http://www.brickboard.com/RWD/index.htm?id=1489605

Double check for vacuum or air leaks. I wouldn't think the TB shaft would be an issue unless it's real loose in the body.

Dan

While all of the new parts you have recently installed might seem like they would eliminate many potential sources of the running problems, in my opinion, they are MORE likely to be the source of the problem than something you didn't replace; that, because either there is a connector or ground not quite secure, or because the new parts have failed. Board members have occasionally found that even good, name brand electronic parts are not what they used to be.

In other words, do not eliminate something just because it is new.

The 'scope discussion is beyond my experience, and possibly overkill for diagnosis, but I bet Art B. could answer those questions.

Check for codes on the OBD module.

Are all the spare parts you swapped in known to be good?

Have you replaced with new bosh or Bougicord plugs, wires, cap, and rotor?

Have you cleaned the throttle body, Idle air control valve, and flame trap PCV system?

Check for air leaks between the AMM and the throttle body. Check for vacuum leaks.

Thats a start
Dan