E420 Idle Mystery

[dproudfo]

I replaced the ignition system on the 1995 e420 this weekend (caps, rotors and plug wires) to resolve a little bit of a rough idle. Car has 88k on it so I figured it was time to nip it in the butt before the wiring problem got any worse. The problem for the most part was resolved and the motor doesn't even feel like it's on now when sitting at idle. (Smooth as can be)

BUT, here's the catch, in the morning when starting the car, the engine will go through its normal cycle where it holds a higher RPM to warm itself up and then drops down to a normal idle at park and that's when I can feel a little 'roughness' from the engine. It is ever so subtle but is definitely there. After running the car for just a couple of minutes everything is stabilized and the engine runs perfect.

Any ideas? It seems like the car is just dropping the idle a little too low following a 'cold' startup. Hmmmm .

Ps. Wiring harness hasn't been replaced but I'm hoping to get over 100k before having to rip into it.

Any feedback is always appreciated!

[deanyel]

The idea is to nip it in the bud. Nobody likes to be nipped in the butt. Are you getting any fault codes?

[jcyuhn]

I used to have an Old English Sheepdog that would nip you in the butt.

Butt I digress. Sounds too subtle to set any fault codes, though it's always worth a look. I would guess something like dirty-ish injectors. When the ECU begins the lean out the fuel mixture at the end of the warmup cycle, if the spray pattern is a bit compromised, the engine won't run quite a smoothly. Try using a good injector cleaner. Or possibly it is just old engine mounts which don't absorb vibration quite as readily as when they were fresh. Seems likely to be an interesting problem to troubleshoot. Most owners don't go after things this small.

- JimY

[dproudfo]

Motor mounts were replaced a month ago so I have that checked off the list. Injector cleaner on the way. I'll keep you posted on the outcome. Also, is it possible that the temp sensor could be bad? Cold start with in improper reading could equate to a fuel mixture that's too lean causing potential misses?

Fingers crossed that the injector cleaner does the trick.

[dproudfo]

Well, she started actually having a few misses yesterday. The missing is as random as it gets. When I'm sitting at idle (both in Park and in Drive) every 30 or so seconds I'll get a "hiccup" from the engine (A solid miss causing the car to shake a little) Also be advised that sometimes the car will go 5 minutes without having a hiccup at all. I'm planning on running Techron later tonight to see if it fixes the problem. Any other thoughts? Do bad sensors cause intermittent random missing? (ie; temp sensors?)

[deanyel]

You've checked the fault codes?

[dproudfo]

Originally Posted by deanyel

You've checked the fault codes?

When it was so subtle before I figured that it wouldn't be enough to trip a fault. Planning on the running codes tonight or tomorrow.

[Mike Freeman]

Here are a couple of things you can check with basic tools.
1-The fuel pressure regulator is located on the fuel rail over cylinder #8,
pull the vacuum line off and plug,with engine running check for any fuel from the vacuum nipple. If any fuel leaks replace regulator!
2-Pull one or more plug(s) and check that they are not resistor plugs. If they are replace them.
3-If you have access to a vacuum pump apply vacuum to the EGR valve,
Pull the line off quickly and listen for the valve to snap shut (metal on metal snap). If not your valve may be bad.
Any of these can cause poor idle conditions.
Mike

[dproudfo]

Well, I ran the diagnostic tonight on the car and the read came up clean. Now on top of dropping a cylinder at idle, the engine is now missing during acceleration as well. (especially when powering over 2500 rpm) In regards to this weekend's install, all components I installed were Bosch minus the spark plugs which were NGK. I'mssuming there's a bad spark plug but considering I double checked the gap on each plug prior to install I still have apprehension about the problem being that easy of a fix.

I've read that some owner's adjust the gap distance on the spark plugs?

I'm in the dark now and it sucks. Keep me posted.

[dproudfo]

One more detail, I've had the car for the past 4,000 miles and the car has never 'dropped' a cylinder like this. It just started 2 days after doing the ignition system.

[deanyel]

"Ran the diagnostics" means all pins or just the DM?

[dproudfo]

I'm using a CS1000 scanner and am just wondering what the main difference is between digital and analog reading? Manual basically says, some systems will respond to a digital reading while others will not?

I'm second guessing my initial analysis of the DM read.

[deanyel]

You should be analog and OK. You want to get the codes from 4 - LH FI, 7 - CC/ISC, 8 - Base module, 17 - dist. ignition, and 19 - diag module. In other words in flash code reading you want to get only a single flash - no faults - from each socket.

[dproudfo]

Just finished reading the codes and came up with nothing other than a DTC #5 on socket 7 which reads a Stop Lamp Switch failure? All other sockets came up clear.

Side note, I have plug-ins for sockets 13 and 14 (copper lining) but can't read them, is that normal?

Keep me posted.

[dproudfo]

In terms of where to go now, I'm leaning towards pulling the covers and checking each cable and connection for the plug wires and spark plugs. Are there particular areas that I should look for that would be potential problems? (ie; cables crossing in the wrong places, spark plugs, etc.)

I'm wondering if there's a lose spark plug connector somewhere or a loose fitting on an End Plug.