328 Slow down light

You could pull the plugs and examine the electrodes for an indication. Any general purpose shop manual should have pictures of spark plugs indicating different things like misfires, too cold plugs, etc. But if it's just one plug, it might be as simple as "Which of these things is not like the others?"

But the problem might also be an intermittant leak on the cold start injector, if you're smelling gas.

Is there a corresponding reduction in performance when the SD light comes on?

If you only get this at constant (low) throttle, and revving the engine makes it go away, your AFM plate might be sticking.

 

DGS,
I know you are quite familiar with the various electrical circuits/components on the 328...can you describe what are the various "trigger inputs" that set off the "slow down" light?

I know on my old 308, it was just high cat temp, I surmise it is more than just that on the 328....

 

There's a bit of a complexity on what is presumably an "enabling" loop with the ECUs (the Bosch system doesn't go closed loop until the cat reaches 300C).

But the "slow down" light is just cat temp driven. warning (flashes) at 900 +/- 20C, error (steady) at 940 +/- 20C.

The air flow metering (AFM) plate sits in the airflow and deflects downward as incoming air pushes it aside. That's how the CIS system figures out how much fuel to give the engine. As the revs slow and the engine uses less air, the control pressure pushes the plate back up. If the plate sticks, the CIS may be feeding too much air to the system. If your control pressure is low, the airflow would deflect the plate too much, and again you'd get too much fuel. But a sticking plate is the kind of thing that would be a sporatic intermittent. When the control pressure regulator (a.k.a. warm-up regulator) goes bad, you get lousy performance at different engine temperatures (cold or warm).

You'll find the circular AFM plate just under the air filter box. Be careful fiddling with it, as it's delicate.

 

Hi Russ,

Picture of my decidedly non-factory (Craftsman) 328 sparkplug removal apparatus. Have tried multiple versions & this seems to work best for me. Consists of 3/8" 18mm 6-point sparkplug socket with rubber insert, universal, short 3in extension, and very short extension for some of the 5-8 bank plugs. 1-4 plugs are simply long extension + sparkplug socket. Don't know total price (i.e. with exception of sparkplug socket, all from my toolbox) but total <$20. Note some of the newer sparkplugs (Champion?) don't use the 18mm size.

Carl
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The rubber insert doesn't really protect the plug. It keeps the plug from falling out of the socket, so you can (a) remove the plug with the socket, and (b) insert the plug with the socket (and extension).

Given that you can't really work the plugs by hand on these cars, it's pretty handy to have. But it's not to protect you from twisting the plug wrong.

My set-up is similar to Carl's, except that I don't use a U-joint adapter. I found an extender that reaches just to the top of the valve cover, so I can fit a ratchet handle on it and not have to worry about sideways twist on the socket. On the forward (firewall side) bank, it only just clears the firewall (with the ratchet handle removed), but just enough is still enough.

 

The AFM test was if all the plugs looked alike (problem across all cyls). For future reference, the test would be to gently press (with even pressure around the plate) the AFM plate down and release, to assure that it doesn't stick somewhere in its travel.

If you're getting spark current to all cyls, but one plug is getting fouled by some liquid (maybe oil?), then you still might not be getting spark from that cyl, as oil deposits could short the plug.

I gather "no help" meant that you got the "slow down" light after changing the plug. Did you pull the new plug to see if it was also fouled like the old one? If the new plug is again "black" and covered with moisture, then your problem may be that cylinder.

If that's the case, then you probably need to identify the moisture on the plug. I can't be much help there, as I can't rely on smell so I have to try to identify liquids by feel, and that's hard to describe.

Coolent on the plug might indicate a head gasket. Oil might be caused by rings. A thick fuel residue (rich cyl) might be a problem in the fuel distributor, or you might have a clogged idle pitot hose. (Does it miss at idle?)

From your summary, I'd almost think that you might have a clogged idle air pitot, causing a rich cylinder at idle, and idling at the test station fouled one plug, causing subsequent problems. But replacing the fouled plug should have helped ... at least, for a while.

If the new plug is still clean and you're still getting the slow-down light, then you may have a more general problem -- possibly control pressure, as it sounds like the issue is no longer really intermittant.

Give me a computer problem, and I'll spank it and put it to bed without supper. But these mechanical CIS systems are annoying, as there are no self diagnostics.

 

Russ the symptoms that you're describing sound like you've got a bad slow down ECU. The slow down ECU is located in the trunk on the passenger's side near the antenna. A properly operating system would have the slow down light come on and start blinking when the cat temperature reached 900 C +/- 20 C. The slow down light blinks faster as the cat temperature rises. The slow down light stays on once the cat temperature reaches 940 C +/- 20 C.

If you're seeing the slow down light go from off to full on very quickly (without much or any blinking) and then go off pretty quickly then you might have a bad ECU.

One way to isolate the problem would be to run a pair of wires from the leads coming from the thermocouple to a digital volt meter in the cabin and monitor the voltage coming from the thermocouple while you drive. Doing this won't be as accurate as a proper thermocouple circuit but will be good enough to let you know if you've got an overheating CAT or an ECU problem. Using phone wire works well as the small solid wire allows you to insert a wire into the connector coming from the thermocouple to the slow down ECU and then plug them together (i.e. tap into the connections). The slow down ECU has two sets of connectors; a pair of small round plug connectors and a four pin connector. You want to tap into the small round connectors. They simply pull apart and push together.

You should see readings less than 35 millivolts while you're driving. If you see readings above 35 mv then you probably have an overheading CAT and you should get that problem fixed quickly. If you see the slow down light come on while readings are less than 35 mv then you've probably got an ECU problem.

Good luck and please post what you find.
Jim V.
'89 328 GTS

 

Hi Russ,

Have been following this thread with interest but refraining from contributing as until now haven't been able to offer anything others haven't suggested.

I'm posting this only for info, don't think this is your problem.

Adjusting idle mixture on 328 is done by removing (pulling out/drilling) small metal slug on air meter. Turning CCW leans, CW enrichens. If the car is rurnning OK at idle would not touch. The other site has nice pictures of this being performed on TR. To do this properly you need a gas analyzer (CO minimum) to read pre-cat %CO; I've read described "by ear" going lean until engine begins to slow then turning CW 0.25 turns.

The 328 cat "computer block" is notorious for going bad, and not always predictably. PO on my car had intermittant SD light that "went away with high-octane gas" but FoH simply replaced at major service. I *have read* that others have experienced rough idle when begins to malfunction, but disconnecting mine from input/output to Bosch ECU made no discernable difference (but wasn't running all that properly to start with! -smile).

Now to test TC as I recall there's a thread in archives involving putting specific resistor in series. I would also suggest here that there's a silver tower-like relay that controls/protects O-2 circuit in 328s also "known to go bad". It's a Porshe 928 relay, Bosch 928.615.124.000, for about $153. You're welcome to borrow mine when returns in a few weeks to try out if you'd like.

Hope this helps somewhat. Again, if your problem is intermittant it may be fuel-related but likely *not* baseline mixture setting.

Carl

PS If you want to artificially "induce" a rich mixture unplug your O-2 sensor. Car will revert to "open-loop" FI control.

 

On a 328, the base mixture adjustment is performed with the O2 sensor unplugged. The car "normally" operates open-loop (a) when cold, and (b) at wide open throttle (WOT).
With the O2 sensor unplugged, the frequency valve should operate at a flat 50% duty cycle. (60% when cold or at WOT (for an F105C engine (US); the F105E runs 65%))
If you look between the air filter box and the side wall (under the trunk lip), you'll find a couple of wires with a big connector (spade lug connector with a large rubber boot). That's the connector that gets unplugged when doing base mixture adjustments.

According to SB 80-29 (CIS/lambda electrical tests), the system only operates closed loop with (a) water temp > 59C, (b) oil temp > 25 C, and (c) catalyst temp > 300C.
That last figure is a bit of a mystery, as the only CCU (catalyst ECU) link to the lambda ECU is pin four of the CCU four pin connector.

The idle part of the throttle switch grounds at idle. That switch is directly connected to the Marelli ignition computer pin 19. The switch is also connected through a diode to the lambda ECU pin 6. Pin 4 of the CCU is also connected to the lambda ECU pin 6. I haven't gone probing with a meter to figure out exactly what the mechanism is on that connection. I suspect that the CCU keeps pin four grounded until the cat reaches 300C, but it might be providing pull-up voltage above 300C. Since people have reported that they can remove the CCU without apparent problems (e.g. when installing an aftermarket exhaust without a cat), I don't really think the CCU is providing the pull-up for the throttle switch.

If the CCU is malfunctioning, you might be in open loop all the time, or in closed loop as soon as the engine warms up. O2 sensor outputs tend to drift in a cycle. With an older O2 sensor, you can feel the 328 "surging" at a constant cruise throttle setting (say about 40 mph in 4th) as the O2 sensor cycles. That might be interpreted as "roughness" in the engine. (Check your O2 connector, as a number of owners leave it unplugged because they don't like the surging. The shops are supposed to leave it connected.)

Unless you're dumping raw fuel into the exhaust at idle (possible problem with the cold injector), I'd have to agree that it's looking like a bad CCU module, at this point.
Checking the thermocouple voltage as Jim V described is probably your next test.

If you pull up the carpet at the right side of the trunk (near the ant), you'll find a plate bolted down. If you unbolt that plate, on the under side you'll find the lambda ECU, the CCU, the lambda protection relay, a couple of relays controlling the CSInj and air injection cutoff, and the diode mentioned above. (A similar plate on the left side of the trunk holds the Marelli computer). (If you've ever tried taking ice cream home in that trunk, you'll have noticed that it's not the friendliest environment for electronics. )

 

I have been reading the post with interest as the slow down light came on in my 84 308 QV on Saturday for the first time ever, after about 100 miles of 50-70 mph driving in 85 degree temp, when I was sitting in traffic. No flashing, it just stays on. I park the car, let it cool off, the light comes on when I start it, then winks out after a few seconds, then comes on again about 5-10 minutes later after the car warms up completely. I had to drive the car about 100 miles home, it ran flawlessly. I checked the cat, no glowing, no unusual heat that I could discern.

Another idea is to drive the car for a while after the light comes on, then use a handheld laser temperature probe to check the temp of the cat versus the suggested limits in the manual (you can see the cat if you crouch down and look behind the rear wheel). As I understand it, the cat can melt internally, this plugging the exhaust and stopping the car. That kind of heat would become apparent on the temp meter.

After reading the post, my thoughts are that the problem on my car is either in my cold start circuit (the car hunts and can stall when I start it from cold after a few seconds the idle rises to normal and it runs fine) or in the slow down ECU, as there is no flashing as the temp in the cat rises and the light comes on sometimes when the car is cool, which would make no sense if the cat is not yet up to temp.

The thought that the ECU is affected by engine heat, during prolonged drives on hot summer days also makes some sense. I drove the car on Sunday morning for about 40 miles and no light came on. I parked the car for a while (letting the exhaust heat soak in) and presto, the light stayed on for the drive home.

I guess the idea is to fix the cold start circuit first, see if the problem persists, and then go after the ECU. If all else fails, I don't need to pass emissions in my jurisdiction as my car is 20 years old, so anyone know where I can buy a decent test pipe and get rid of the cat/ o2 sensor and ECU altogether?

I am going to take the car to my tech this week, if we get to the bottom of it, I will post.