hesitating 86

[avt007]

Well, I finally got my first 911 and I love it. But, it has a problem I could use help with. The symptoms are; with steady throttle at any speed , it behaves as though I am moving the throtlle slightly, back and forth about once a second. On light to moderate acceleration in any gear, it accelerates, stumbles once or twice then accelerates some more, stumbles, etc. Above 4000 rpm it seems ok, and heavy or wide open throttle has no hesitation or stumbling. It pulls strongly at higher revs, and sounds great. I did find a vacuum leak, but it made no difference. The way it oscillates in steady cruise makes me suspect fuel pressure, or maybe a really wonky air flow meter. Any thoughts? It's a USA model Carrera Coupe, totally stock. BTW, if anyone has a good picture of the assorted vacuum hoses and where they all go, that would be great. Thanks, guys!

[wolf]

indeed air flow meter, DME.
great car, great year.

[PatrickB]

I've been chasing this problem for quite sometime myself... To no avail...
There have been many different threads and opinions on this topic. Do a search on this BBS, and you will find many different ideas...
The most common approach with your specific issue typically involves replacing several sensors... Head Temp. sensor, flywheel reference sensor, speed sensor, and even the DME relay. If these sensors have never been replaced, it's not necessarily a bad idea to replace them as a preventative maintanence item....
As a quick fix to remove most of the symptom, you can always temporarily disconnect the O2 sensor. This will richen the mixture, deleting most of the lean miss symptom, but your fuel economy will suffer...
Please post your results if you have any success, as I would love to find out what your fix to the identical problem I'm having is!

[kqw]

Start smple.

The check the condition of your plugs, especially the gap. Should be at .028 and running the correct type of plug.

Did anything turn up on your PPI?

Pop the dist. cap and look at the condition of the rotor, inside the cap etc.....make sure there's no play in the dist. shaft.

[john walker's workshop]

it's probably the oxygen sensor doing it's normal adjustment thing. a rythmic surge. unplug it and go for a drive. the farther out the mixture setting is, rich or lean, the more you will feel it. i usually set the CO around 0.8-1.0% with the sensor unplugged and then plug it back in. on 80-83, most people leave it unplugged for the same reason, because it's more of an add-on system, but it's more of a built in system on carreras. early carreras seem to tolerate running unplugged, with no wierd effects, more than say 86 on.

[Superman]

Yes, it sounds like a mixture problem. Listen to John. Find someone with a CO meter and do what John says.

[88911coupe]

My car had same symptoms and, on JW's suggestion, I replaced the O2 sensor...problem solved.

[avt007]

Thanks for al the ideas, guys. I'll try the O2 sensor idea as soon as I get her on the road. Because I bought an out of province car, I have to get through an inspection, and before that, I'm buying tires. So it might be a bit before I can try it. We have annual emission tests here, so next year it'll have to pass a test.

[twin plugged targa]

you may even want to try cleaning the o2 sensor I did this with sand paper and it seemed to help- or maybe someone will correct me on this as BS?
rgds Ben

[avt007]

The latest; my car does have a new style CHT probe. So I unplugged the O2 sensor and tried it, and the car is transformed-it runs like a dream, no hesitation, lots of power and throttle response, etc. I wonder if there is a drawback to driving without the sensor, but is runs great. Should I just buy a new sensor, or investigate more? And for a sensor, what supplier or brand? And last question for now, how rich does it run this, will it mess up my mileage a lot?

[PatrickB]

Possibly 15-20% less mpg, of course depending on how hard you hit it...

[PatrickB]

With the O2 sensor removed, the problem will likely return, however, be far more subtle... (my experience.) Also, you may notice a substantial drop in fuel economy, as i did...
Try speed sensor, and flywheel reference sensor next, and let us know if that worked.... Some may say it's a waste to replace those items, however, if they are original in your 19 year old car, they WILL eventually fail in the future if they haven't already!

[WydRyd]

As I've said before, removing the O2 sensor WILL affect your economy significantly, as it richens your idle and part throttle mixture.

With it connected, it forces the ECU to maintain the A/F ratio in the Stoich region, or 14.x:1 ratio.

[PatrickB]

Stoich region? is that near The Netherlands?

[avt007]

Obviously the system is not maintaining the mixture in the correct range, so what is the fix? A new sensor, a mixture adjustment? A reprogrammed ECU? I don't believe in the shotgun method of repair, where every component is replaced. I'll have to research this a bit more I guess. Thanks, and keep the tips coming....

[Fl Ted]

Had a similar problem with my '86. It was the CHT probe. Easy to check.
Got the fix on this BB. Get a 1/4 watt 270 ohm resistor ( was supposed to be 300 ohm, but thats not a standard size) Shorten the resistor leads to about 1" and bend them over double. put it in your pocket. Locate the cht plug in the engine compartment. Drive the car until it's warmed up and the hesitating starts. Pull off the road and shut off engine. Disconnect the cht plug from the connector on the left side of the engine ( metal bracket with 3 connectors on it) and insert the resistor leads into the connector on the bracket. (wires go to DME) Locate and trial fit all this before you do the test drive. With resistor in place, resume your drive and see if this cures your problem. The improvement will be instaneous if this is the culprit. I drove mine with the resistor in place untill I got the new CHT probe, the only problem was it ran a little rough untill it warmed-up, as the 270 ohms was telling the computer that it WAS warm when it wasn't, resulting in a lean mixture when the engine was cold. When this sensor goes bad (usually intermittent )it causes the dme to change the mixture when it shoulden't, causing the engine to "hesitate", or "buck". Mine was so violent , it would throw me toward the windshield!!!
Good Luck !
Ted

[Hugh R]

F1 Ted

Great write up, which of the three plugs is the CHT?

[Fl Ted]

Hugh: on my car, it's the top one.
CORRECTION; in my previous post, I stated that the resistor was to be inserted into the connector half thats on the engine bracket.
WRONG!, it's the other half, the part that you disconnect from the engine mounted bracket.
Sorry 'bout that!
Ted

[avt007]

Thats good info, I'll give it a try. As I said before, mine does have the probe, but it's worth checking. The O2 sensor looks suspicously clean too, so it is probably fairly new as well.

[PatrickB]

any luck yet avt?