Surge, Rev, Die, Now Won't Start. Clues?

[Smoove1010]

The Porsche gods were looking over me - she died just as I turned into my driveway after a 45 mile commute home! I'm hoping the symptoms leading to the stall will "speak" to someone and give me a head-start on troubleshooting.

87 3.2 with just 500 miles on a new top-end job. Wires, cap, rotor, plugs, pulse sensors, fuel filter are all new. Fuel pump has about 3K on it.

Since the top end project, she's been running great, idling steady, starts right up. Drove 45 miles to work this AM, car ran fine. I have the O2 sensor unplugged as part of troubleshooting a mild case of the low-rpm herky-jerkies.

Started right up after work, ran fine most of the way home, but with 5 miles to go I noticed at stop lights that it was idling high - about 1100 RPM. A few more stoplights later, the idle started oscillating from 900 - 1200. Surging during acceleration and cruising became pronounced. With a mile to go, while at a stop light, the idle started swinging from about 900 to nearly 2000 RPM. Noticable bucking during acceleration. Just as I pulled into my driveway there were a few "revs" as though someone was stepping on the gas, then she stalled. I hit the key a few times, the starter turned the motor over just fine, but no start. I tried a few more times and gave up once I smelled a faint fuel odor.

I'm guessing ignition, and will start there with my troubleshooting, but I'm hoping someone sees this and recognizes these odd death-throes as classing symptoms of something someone more experienced than I knows about. Because of the fuel smell I'm guessing it's not the DME relay, but could it be the DME? Would a coil failure occur like this? Lots of failures could cause an engine to suddenly die, but these "last gasps" and phantom-revs have me stumped...

Thanks as always in advance for any wisdom!
GK

[Smoove1010]

Uh oh, no takers - is this occurrence that bizarre?

GK

[Smoove1010]

It gets even more bizarre: I re-connected the battery, hit the key, and it started right up. Hesitant to drive it until I figure out what's going on. Could it be the DME relay or even the DME mis-behaving after it gets warmed up? I'm inclined to put in a new DME relay, them perhaps take the old one apart to look for cracked solder joints.

Am I on the right track?

Thanks,
GK

[HawgRyder]

The "hunting" of the engine sounds like mixture (fuel problems).
If the fuel goes lean...the RPM's will rise.
Is it possible that the filter inside the tank is dirty?
If the pump is running...but the fuel supply is not consistant...this would cause the exact set of circumstances you have.
Bob

[docrodg]

Probly filters, possibly some pressure issues too, but start with the filters

[mhicks85]

I have an 85 which used to do the same thing. When the car was hot the idle would go everywhere. I had to adjust the idle bypass. It adjusts the starting idle mixture from which the computer has some +/- room to control the idle over all conditions. If the idle bypass adjustment is to high the the computer will not have enough range of control.

If you do a search you will find the procedure.

[Smoove1010]

Originally Posted by mhicks85

I have an 85 which used to do the same thing. When the car was hot the idle would go everywhere. I had to adjust the idle bypass. It adjusts the starting idle mixture from which the computer has some +/- room to control the idle over all conditions. If the idle bypass adjustment is to high the the computer will not have enough range of control.

If you do a search you will find the procedure.

Mhicks -
Interesting thought. I did do this procedure prior to the top-end, but it's reasonable to assume that it needs to be re-done since substantial changes were made, including reman'd injectors, change in valve timing, etc.

It just seems odd that she would idle solid and steady for 500 miles, then exhibit this bizarre behavior before going flat-line. Because it started up readily after cooling down over night, I'm still thinking there's a component that's getting flaky when warm.

It's got the slight herky-jerkys lately in the 1000's and 2000's but takes off like a bandit over 3K, so I'm thinking it can't be a fuel filter or other restriction.

Thanks,
GK

[Smoove1010]

Because I've spent a lot of time reading trouble-threads that too often don't close the loop and end with a resolution, I wanted to close the loop on this one.

Symptoms started out as noted above, then continued intermittently. I got a new O2 sensor from Amazon and a new DME relay from our host, installed both on Thursday night, then started the car and let it run for about 30 seconds. The next night I went to take her for a ride, she barely started, blew a bunch of black smoke and sounded like she was running on 3 cylinders. She died and wouldn't start again.

Spent an hour or two surfing threads, and CHT kept coming up as a culprit, but I was satisfied that mine was OK as I had tested it during my top-end project.

BUT, I was going back to basics today, checked the idle and WOT switches, and power at those plugs; checked coil voltages and resistance, checked for spark, was about to pull spark plugs when I noticed that the white CHT connector was not pushed fully into place - today's Eureka Moment!

Once I pushed that connector firmly into place, she started right up. I went for a little cruise to warm her up, then followed the base-idle setting procedure to make sure all was right given all the recent changes.

Conclusion - a properly working CHT sensor only does so when connected! Mine was apparently losing connection intermittently, causing intermittent symptoms of a failed sensor. It all makes perfect sense.

Shout-out to kidrock for suggesting CHT - he wins closest-to-the-pin on this one!

The new O2 sensor has improved performance, throttle just off idle, and the mild surge/flat spot during acceleration is about 90% gone. Next - re-check that AFM. I'll get rid of those flat-spots yet...

GK

[kidrock]

Way to go, GK. Glad to hear you're back on the road.

[stlrj]

Originally Posted by Smoove1010

The new O2 sensor has improved performance, throttle just off idle, and the mild surge/flat spot during acceleration is about 90% gone. Next - re-check that AFM. I'll get rid of those flat-spots yet...
GK

I've spent a lot of time on my 87 AFM to get rid of flat spots and found that backing off on the CO adjustment screw to lean out the mixture works like magic. It seems the 02 sensor controlled DME system operates optimally in a fairly lean environment.


Cheers,

Joe

[Smoove1010]

That's a thought Joe, thanks. I may give that a try with the notion that I'll keep careful track of the number of turns I make so that I can put it back where it was if it doesn't work out to my satisfaction.

GK