Ignition issue - any thoughts?

[Porsche-O-Phile]

Some of you might've seen my post in the classifieds looking for a DME - I still haven't ruled out a DME failure here.

Here are my symptoms:

- Car has been driven very sparingly in the last 6-9 months following a successful belt/waterpump job and retension. It ran fine after the retension although it was driven very infrequently thereafter. I recently started it up and noticed poor idle and extremely poor performance overall (misfiring, lack of power, difficult to get the engine to rev up, even in neutral, etc.) Another odd thing is I think the tach was reading low (i.e. the "sound" would tell me 3,000 RPM and the tach would indicate 1,500 or so, etc.)

I suspected ignition, but initially (just to be sure) ran some seafoam through the gas tank to clear up any sticking injectors. No improvement. Still a lot of sputtering and poor operation.

I pulled the coil - resistance between the + (black) and - (green) terminals is a consistent 0.8 ohm (normally supposed to be 0.4-0.6, so it's a little high but probably not a show-stopper). Resistance between the - terminal and the output is 5,300 ohm - right where it should be. So the coil is likely okay.

Pulled the spark plugs out one at a time and tried cranking the engine. I observed only a (very weak) spark on the #1 plug, nothing at all from the remaining three. Condition of plugs overall is okay, although some carbon (probably from running it with intermittent ignition). Compression is fine in all cylinders. I pulled the cap off the end of the fuel rail and cranked - fuel is flowing fine (so the pump is pumping).

Sounds like ignition only. What else should I check? Speed/ref? Any way to check these without blowing $500 on an oscilloscope?

Am I forgetting something obvious here? Just want to run it by the PP brain trust before I blow $$$ on a spare DME and/or oscilloscope.

[josephsc]

Hey POP, good to see that you're still hanging around these parts.

I'd say definitely check the speed and reference sensors as well as the DME -- especially if the tach seems to be doing strange things. If the tach is reading lower than true, then it's either not getting good signal at the sensors, or the DME isn't passing the information along.

And rather than buying a DME, surely you can borrow a working DME from someone, no?

[Porsche-O-Phile]

What up Joe?

I'm looking around to see if anyone has a spare early DME. FWIW I have that Link standalone sitting in a box, but I don't feel like making life harder on myself right now trying to install that while I've got a problem (might mask other problems created by the swap). Early car DMEs are a bit of a pain - there are several flavors of them and I need to make sure I get the correct one.

I haven't done the speed/ref sensors before. Is it possible to replace from the top or does it require dropping the engine/trans?

[jwade944]

I have not searched for earlier threads so if this is redundant, please forgive me. The DME looks at the speed sensor to reach a minimum RPM before it energizes the fuel pump. If you are getting fuel, then the DME is receiving (and processing) the speed signal. That does not rule out the sensor (or DME) but lessens the chances that they are the problem. Have you checked the distributor cap and rotor to make sure they are on tight and are dry? Is the cam on mark with the engine at TDC? Is the battery fully charged? (Low voltage to the DME can cause erratic performance.) Are the grounds in good shape? Any vacuum leaks? (Check the large vacuum hoses under the intake manifold.) Is the airflow meter moving freely and plugged in tightly?

To answer your question, it may be possible to replace the sensors without sliding the engine forward, but I doubt it. They are located on the front of the bell housing near the heater control valve. If you can get to them, remove the bolt on top of each sensor and pull straight up out of the mounting bracket. (Easier said than done as heater control valve leaks have probably frozen them into the bracket. Try not to remove the bracket. If you do, you will have to reset the height so the pin on the flywheel does not collide with the sensor. (Not the end of the world, but one more PIA.) Make sure to mark the wiring connectors behind the intake manifold. The sensors are identical, but the engine will not start if the wiring is reversed. The connectors are old and brittle. Be very careful taking them apart.

[Slam]

Cap? Rotor? I'm a fan of simple and easy first.

[HondaDustR]

^ +1. Cap and rotor points could be shot.

[Porsche-O-Phile]

Yea, forgot to include that - I did check the cap/rotor. Put the engine to "OT" #1 TDC and pulled the cap. Rotor wast pointing to approximately the 10-o'clock position. I set that plug wire as #1 and then moving clockwise around 1-3-4-2 just as stamped on the cam tower. So I'm fairly certain the rotor is pointing to the correct location(s) at the correct time.

Everything was dry/clean inside (I cleaned it anyway). So 99.9% sure the cap/rotor isn't the problem.

[KitCarlsonEMS]

Whoa! If the rotor was pointing at 10-o'-clock you need to spin the crank one more revolution. It should be near 4 or 5-o'clock! Please reset those plug wires! The distributor turns 1/2 the engine speed. The 10-o'-clock position will fire on exhaust!

[VINMAN]

Jeff,
you can get at the sensors from up top. Not that easy but you can do it. I used a long extension with a swivel socket. Only real problem was getting them out from the bell housing because they were corroded in. I ended up breaking one trying to pry it out.

[944 Ecology]

Porschephile, you have PM.

[HondaDustR]

If you counted cylinders from back to front, the furthest plug from the front should end up at the 10 o'clock position.

I'm pretty sure the plugs are commonly counted front to back, in which case the real #1 wire will go to the 4 o'clock post, followed by the 3rd, 4th, and 2nd wire clockwise from #1 on the distributor cap.

[hpservertech]

Have you checked all your vacuum lines yet?

Speed/ref sensors can be taken out easily from the top. You dont have to go and move the engine or drop it. Just unbolt them and pull them out. Should be no more then an hour to do it.

[hpservertech]

Have you checked all your vacuum lines yet?

Speed/ref sensors can be taken out easily from the top. You dont have to go and move the engine or drop it. Just unbolt them and pull them out. Should be no more then an hour to do it.

[Porsche-O-Phile]

Checked the vacuum lines. I will double-check the OT mark for the rotor location. That's precisely the obvious head-slap kind of thing I was thinking this might have been about. . .

Very cool if all it is was the rotor being 180-degrees off due to 2-for-1 rotational speed versus the dizzy.

Cripes. This is why I shouldn't work on cars when I'm overtired (like I was the other day).

I'll go look at it tomorrow. I have a funny feeling that might solve my issues.

[Porsche-O-Phile]

Thinking about this a little more - I'm almost certain that's going to be it. I remember griping to myself about the #2 spark plug wire being a bit short to reach the distributor post at the 7-o'clock position. Probably because if the thing is reversed and I got the rotor location marked on the TDC for the exhaust instead of intake stroke #2 should be at the 2-o'clock position, which is easily reachable by the wire.

It still does not explain, however, why the car would just spontaneously go from running fine to running like poop. Although I'm sure this isn't helping things.

[KitCarlsonEMS]

When the crank is at OT, you also want to check the inspection hole for the CAM to be indexed correctly. Else cam belt is off!. The rotor will be near 4 o'-clock.

[many944s]

Sorry I didn't see this until now, I have a spare early DME if you need to borrow one. I'm headin out of town tomorrow, so if you can let me know later today I will get it sent out to you in the morning.

-Nick

[Porsche-O-Phile]

Update:

I ran home at lunch today an reconnected the coil & DME, and switched the plug wire locations 180 degrees from where I'd put them after the diagnosis the other day. Car started right up (so obviously I set "OT" on the exhaust stroke, not the intake stroke previously) but it still runs like crap.

At this point I'm going to ass-u-me that the DME is good and that the coil is good. The symptoms are rough idle and operation (kind of sounds like it's running on 2 or 3 cylinders) and the tach still indicating lower than where I know the idle really is (I'll confirm this with a separate external tach later).

Does this still sound like ignition then? Something is just "off".

I'm probably going to check each of the plug wires for proper continuity later, then the plugs (it can't hurt to clean them - I might just have one fouled), then the injectors if things are still FUBAR.

Thanks for everyone's responses/help. Any additional suggestions on things to check from here on out are appreciated (although at least I have the car semi-ambulatory again).

[KitCarlsonEMS]

If it has Bosch platinum plugs, the electrodes can drop out.

[Porsche-O-Phile]

Nope - I avoid those things like the plague. Stock Bosch. One or more may be fouled though - I'll check and clean/regap/replace as necessary.