In trying to troubleshoot my continuing intermittent acceleration lag (which is resolved by turning off and restarting the car) I have taken a timing light out with me and waited for the lag to happen.

when it finally happened I pulled over and checked the timing, ok at idle, then when I pulled the throttle partially open the timing wouldn't advance but would stop at around 18 degrees advance.

the good news is I think this rules out fuel starvation as the cause of the intermittent lag. The bad news is I have no idea what would cause the timing to go through this and then be reset again as soon as the car is restarted?

maybe it's a sensor somewhere getting reset at restart.

I've switched out the following parts in order to see if they would change the problem:

1 - Distributor
2 - ignition wiring harness (along with plugged in ICU)
3 - ECU
4 - AMM
5 - wire from coil to distributor
6 - Engine wiring harness (had to be replaced anyway)

could it be my coil?

I'm afraid I still haven't completely wrapped my mind around ignition timing but by the time I figure this out I think I'll be an expert.

any ideas would be appreciated!

thanks,

jack
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'86 245, 283,000 mi., '87 760T engine compartment burned to a crisp

Glad I caught this post. I`ve been having the same problem with my `86 245. There are times when its accelerating poorly, when driving next to a Jersey barrier or anything else that reflects sound, I can hear the motor chatter like its starved for something. It doesn`t blow smoke or anything, and there is no carbon inside the tailpipe either. So I figure its running lean and I`ll check the other stuff suggested on this thread. Thanks to all.

I had (and still have) the same problem with my 88 240. Took several tries to pass smog because the timing would shift (see my post entitled Ignition Timing -Phantom Shifting). Aparently what my mechanic and I eventually found out, through massive research efforts, is that the plugs for the TPS and IAC are roughly the same form factor and were accidentally switches, sending +12V where the ground should have been. This caused a specific circuit in the ECU to short out, sending faulty information to the ignition computer which in turn would either advance or retard the spark as it damn well pleased.

The resolution was simple once I managed to pass smog - My timing was fluctuating between 12 BTDC and 18 BTDC... I wanted it more toward the 12 side of things so I retarded the timing a bit so now it runs between 9 BTDC and 15 BTDC... and runs perfectly. Give it a shot, what do you have to lose?!

--
-Flup, 240DL Wagon, 2 subs in the trunk, Dancing Bears Sticker to top it off :)

"In trying to troubleshoot my continuing intermittent acceleration lag (which is resolved by turning off and restarting the car)"

IMO, your Volvo/Chrysler Ignition system is perhaps the most reliable thing under the hood. I think your fix (turning off and restarting the car) is leading you astray -- and it could be temporarily "fixing" a catalytic converter problem, rather than doing anything to the ignition.

I had an '87 240 whose cat "guts" came loose and, with time and vibration, the debris formed a compact ball of scrap about 2.5" in diameter.

Under varying conditions of accelleration and terrain, this "cat ball" would roll back and plug the cat outlet, producing much the same symptoms as you describe.

When the car stopped (or sometimes under braking deceleration), the ball rolled forward again, freeing the exhaust, and all was well till the next time.
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Ok,

I will consider anything...

so is the best way to check the catalytic converter to remove it and see what's in there?

or would there be a way to test it as it remains on the vehicle?

thanks,


jack
--
'86 245, 283,000 mi., '87 760T engine compartment burned to a crisp

I would first consider the age. In my case, it was original and at least 10 years old. Maybe 150,000 miles on it or more. If yours is that old -- or a $99 replacement more than 4 or 5 years old -- it would be suspect.

It's been several years, but I think we could hear the debris ball bounce when thumping up on the bottom of the cat with a rubber mallet.
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

I had the exact same problem with my '88 244GL, it drove me nuts until I figured it out.

Ignition timing is set when the engine is at idle, and is advanced by changes in a control system, which system depends on which car you have.

Your timing at idle should be 12 degrees before TDC. Your check with the timing light should have shown that. The ICM computer sytem will cause the timing to advance according to engine speed and manifold vacuum (indicates engine load).

The manifold vacuum gets to the ICM via a white nylon tube that begins at the intake manifold with a 90 degree rubber elbow, runs down under the front of the engine along with the wiring harness, and connects to the ICM. The ICM has a little device on it that looks like the same things that control the flaps in the cabin heater - a/c air directions.


Check that you are getting vacuum at the ICM connection. If you pull the tube off the ICM, you may 1) kill the engine from idleing, and/or 2) have a hard time getting the tube back on. I've never done it. Blow through the tube to be sure it's clear, or see if there is vacuum at the ICM end of the tube when at idle.


A word about spark advance. It's a matter of being able to fire the mixture in the combustion chamber at the right time relative to the position of the piston.

The mixture in there does not explode, as many think. It burns, it has a flame front, and takes time from ignition to reach its maximum expansion.

The engine gets best power when that maximum expansion is reached when ALL of the expansion can be utilized to push the piston downwards. (Note that if that maximum expansion point is reached too soon, you get a knock because the expansion is trying to push the piston down the backwards way. Thus the knock sensor momentarily reduces the spark advance.)

To get the best use of the expansion, the ignition must begin before the piston up to TDC. How much before? Well, the flame front speed is pretty much constant, but piston speed is not. At idle speed, (750 rpm +/-) the 12 degree TDC setting is OK. As engine (and therefore piston speed) goes up, the ignition must begin farther ahead of TDC.

Thus the need for a dynamic spark advance system. Our LH Volvos use a computer to do that, and believe me it's a lot simpler and more accurate than what was before.

Use your timing light to (1) check that timing is at 12 degrees and steady at idle, and (2) check for amount of advance on acceleratin when you are NOT having the problem.


Long post, I know. Hope it helps with the timing understanding.

Good Luck,

Bob

:>)

BC - was this nylon hose you speak of on an 88? I've an 83, 84, and an 87 -- all of which have regular rubber hose disappearing into the grunge below the crank. Perhaps the nylon was an "improvement" bringing its own vulnerabilities?

Well, I don't really know, now that you mention it. They all have a tube, and I rather expected that they all used the same stuff.

Tomorow, in the daylight, if I remember to, I will look at the 1986, 1988, and 1984 tht are in the driveway.

I know the little elbow used at the fitting on the intake manifold is rubber stuff, and the diameter of a vacuum hose. But it connects to the nylon tube about an inch down.

We'll see.

Regards,

Bob

:>)

Bad knock sensor?

that might be it... I'll try to test it
--
'86 245, 283,000 mi., '87 760T engine compartment burned to a crisp

Just a loose terminal on a good knock sensor will mimic ferocious knock causing 6, then 12, then 18 degrees retard.

What about the "power stage" that is bolted to the passenger side in front of the strut tower? I can't tell you exactly what that does, but I thought it was involved in timing advance.
--
Andy in St. Paul. '89 244 153K mi, '91 745 Regina 202K, '88 244 184K

Never knew it existed I'll take a look at it!
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'86 245, 283,000 mi., '87 760T engine compartment burned to a crisp

You don't have a Power Stage on your '86 with Volvo/Chrysler ignition. It comes with the EZ-116K ignition, starting with the '89 240.

You say the advance stops erroneously at 18°. How much advance are you looking for? I don't see any specs in the Bentley manual. I'd look in the Haynes, but there's nobody here to put a gun on me right now.
--
Bruce Young
'93 940-NA (current), 240s (one V8), 140s, 122s, since '63.

Thanks, I looked and only noticed the ICU in front of the strut tower so I've already replaced that ....

it seemed to stop at about 18 degrees this one occasion but I'd be willing to bet that it would stop at a different point in a different circumstance.

the lag can happen at any speed from just rolling off to 50 mph on the highway...

restart and it fixes itself.

jack
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'86 245, 283,000 mi., '87 760T engine compartment burned to a crisp

If there are knock sensors on your engine , replace them, bethca they are the culprits. Check your vacuum hose's, I know people are getting sick of me saying this but they can really cause problems that will drive you crazy if they are leaking.
Hope that helps
Poolman

I do have vacuum at the ICU

i pulled off the hose and it sucked my finger on to its opening

I am gonna try to get it to fail again and see if vacuum is no longer present... though I wouldn't think vacuum would be intermittent????

thanks!

jack
--
'86 245, 283,000 mi., '87 760T engine compartment burned to a crisp