Bucking stumbling problem when cold

[bill3667]

I've done a search and found a few threads that are close to my problem but not exact. I want to know if I'm on the right track or going in the wrong direction.

Awhile back my car would buck when starting off cold, it would and still does start right up w/ first turn of the key, it was only when I took off in gear it would start to stumble / buck. It does not stall as long as I put in the clutch. A couple of weeks ago it did this, I only ran it for a few min. and put it back in the garage.

I decided to drive it this evening, again it starts right up but once in gear the bucking is far worse than it's ever been before so I park it back in the garage. The car idles just fine and 2k+ rev in neutral is also fine, only problem comes up when in gear under load.

I start to thinking what would make it worse sitting in the garage (not being driven) and come up with the battery so I check the voltage.

Car NOT running, at battery = 12.34v
car running at 2k rpm = 13.8v

It's obviously charging but would 12.3 v batt cause this problem? I have the battery on a 2 A auto charger now and could wait for tomorrow to see if that resolves it but would like your thoughts on this.

The usual culprits have all been replaced in the past 5k mi.:

- Cyl. head temp sensor (2 wire)
- O2 sensor
- DME relay (9k mi and 3 years ago) I may put the original back in if battery charge doesn't solve it.
- Full shop tune (plugs, fuel filter ect) 9k mi ago

It actually feels to me like the engine is not getting enough fuel but I don't have a fuel pressure tester. The fuel pump was replaced (leaking badly) 3 yrs / 9k mi. ago.

The car's an '88 911, 3.2L, 57K mi.

Any thoughts? TIA

[stlrj]

There must be a cause and effect. What was done before this started happening?

Joe

[gduke]

Ideas. Try some fuel injector cleaner, check for loose plug wires( have the wires been changed they should be replace before 57,000mi's and you car is 18 yrs old), also, dist. caps and rotors can wear out before 10,000 miles. I change my dist. cap in my old Dodge PU every year and the symptoms are the same as your cars'.

Just a few simple things to check. I always start by checking the simplest, cheapest things and if the problem is too complicated I take my car to someone who knows more than I do.

gary

[bill3667]

Originally posted by gduke
Ideas. Try some fuel injector cleaner, check for loose plug wires( have the wires been changed they should be replace before 57,000mi's and you car is 18 yrs old), also, dist. caps and rotors can wear out before 10,000 miles. I change my dist. cap in my old Dodge PU every year and the symptoms are the same as your cars'.

Just a few simple things to check. I always start by checking the simplest, cheapest things and if the problem is too complicated I take my car to someone who knows more than I do.

gary

I would think if it was injectors it woul occur all the time, not just for a short while after startup?
Plug wires were replaced apx. 6k mi. ago, need to recheck cap and rotor though.
If I can't resolve this w/ battery chaege or returning the DME relay I'll be taking it to our local pro too.

Thanks

[gerard vaglio]

My 79 does something like this but once it warms up, no problem.

For me I think it's the warm up regulator otherwise it wouldn't run so well after it warms up.

Perhaps your year has a warm up regulator equivalent?

[stlrj]

Were the ignition wires replaced too?

[bill3667]

Originally posted by stlrj
Were the ignition wires replaced too?

Are you refering to the plug wires? Plug wires were replaced w/ Magnacors apx. 6k mi. ago.

I thought about the Idle Air Control valve and the cold start injector but it starts right up and idles very smoothly from the start, it's only when a load is put on the car (driving off).

It was running too crappy last evening for me to try and let it fully warm up to see if the problem went away when warm but in the past that's what it's done.

As I said, it's acting the same way it's done in the past only for a much longer time, before it would do this for the first 3 - 400 yds but last evening I couldn't get it out the subdivision.

I keep coming back in my mind to battery charge. When I went to bed the charger had been on the car for about 4 hrs and it was still charging at the full 2A rate ( it's a 10 - 2 auto charger on the 2A setting), this AM it was all the way down indicating fully charged. I'll test my theory this afternoon.

Thanks

[DonMo]

Check your reference and speed sensors. Disconnect your ECU and jumper the pins for 25 and 25 with your volt meter on VAC. You should get a reading as you try to start your car, do the same for 27 and 8. If you see voltage, your sensors are working. It also could be your AFM having a bad spot on low throttle and not getting the right air to fuel mixture.

[ianc]

Check your reference and speed sensors.

If the ref sensor or speed sensor are not working, the car will not start.

I think you're running down the wrong track with the battery voltage thing. Once the car has started, it is running off the current produced by the alternator, so the battery should be out of the picture. Your charging voltage (13.8v) is fine.

It sounds like an initial enrichment problem, although to my knowledge, this is chiefly controlled by the CHT, and the air temp sensor in the AFM. You might want to check the latter of these if you've already replaced the CHT.

Just for grins, try disconnecting your O2 sensor as well, and see if there's any difference in the problem. Theoretically, this should be out of the picture as well for at least the first 30 seconds until the heater warms it up, but it is easy to do.

Make sure also that the vacuum lines to the fuel pressure regulator and damper are on securely and sealing properly, including where the Y connector attaches to the throttle body.

It would be interesting to check fuel pressure and regulator function as well. If you're still coming up empty, you might want to try FR Wilke's mod on your AFM. Search here for posts on it.

Keep us posted on what you find out and good luck,

ianc

[ruf-porsche]

BAD GAS??

[nut11]

Techron. Not trying to sell a product but just try it. Are you close to an oil change? If so, put in a 12 or 20 oz bottle with one half the fuel fill up recommended - maybe a bit more. Drive it like you ..... I bet it solves your problem. I think it's your injectors.

[bill3667]

Originally posted by ianc
I think you're running down the wrong track with the battery voltage thing. Once the car has started, it is running off the current produced by the alternator, so the battery should be out of the picture. Your charging voltage (13.8v) is fine.

This is what my mechanic said when I dropped it off w/ him today. BTW, fully charging the battery did not change anything. He also said he's seen worn distributors act like this, he checked the up/down shaft movement and said mine was borderline and would check it further. He had to replace another one earlier this week and still had it at the shop.

It sounds like an initial enrichment problem, although to my knowledge, this is chiefly controlled by the CHT, and the air temp sensor in the AFM. You might want to check the latter of these if you've already replaced the CHT.

Since this is my thread I get a second guess...I suspect the AFM but I don't have a spare lying around to test that theory, hopefully he does.

Make sure also that the vacuum lines to the fuel pressure regulator and damper are on securely and sealing properly, including where the Y connector attaches to the throttle body.

Did this, several times.

It would be interesting to check fuel pressure and regulator function as well. If you're still coming up empty, you might want to try FR Wilke's mod on your AFM. Search here for posts on it.

I'm aware of it including the testing w/ a 9v battery. Unfortunately, I'm time constrained. The car must be drivable by next Fri. so I gave it to my shop to resolve it.

Keep us posted on what you find out and good luck,

ianc

I will update the post with the shop's findings, hopefully it won't be too painful (or embarising).

[bill3667]

Just for S&G I checked the price here on Pelican for an Air Flow Meter (Mass Air Meter) and running through P's catalog it shows p/n 964.606.050.x. with a price of apx. $1200 (WTF?). When I click on the Photo link it shows the correct part.

I then check PET and the p/n shows 930.606.123.00 and PET plainly shows the part, when I search on that p/n on Pelican it brings up Air Flow Sensor w/ a price of $440 but no link w/ a photo. I'm confused, am I missing something? I know 930 is turbo but most parts are interchangable, is the AFM?

If it came down to needing the AFM I'll call, but I suspect my mechanic has his own parts sources. It just has me curious.

[stlrj]

Plug wires were replaced w/ Magnacors apx. 6k mi. ago.

Is it possible that the symptoms you describe began after you installed the Magnacors?

[bill3667]

The cause of the problem turned out to be the Idle Position Switch (microswitch). P/N 911.606.013.00

One of the functions of this switch is when you decelerate (let off the throttle) this switch tells the DME to shut off fuel to the injectors so fuel isn't dumped into the cyls. when you're slowing down, and turns the fuel back on when the RPM's get down to apx 1080 RPM.

This is why it acted to me like a fuel starvation problem (as in my orig. post) and why it would idle just fine.

Lesson learned, check ALL the easy items even if you just checked them 4 - 5 months ago.

Hope this helps someone else sometime.

[ianc]

I'm not understanding why the idle position switch being misadjusted would cause the symptoms you describe.

Your original post described bucking and stumbling only when moving off in gear, but that the car idled and revved OK, but my knowledge of what the IPS does is not consistent with your original complaint.

What was wrong with the switch? Can you elaborate?

ianc

[bill3667]

The switch wasn't mis-adjusted, it was bad. I can only summise (sp?) the output from the switch is not looked at by the DME (which turns off the injectors) all the time. I think this makes sense since any time the throttle is pushed and the Idle Position Switch is opened (?) you wouldn't want the DME turning off the injectors, you would only want the injectors turned off during decelleration.

Edit: I think I got this backwards, the DME would turn off the injectors when the switch is closed (totally off throttle) but NOT sitting at idle. ??

This explains why when it started bucking and I pushed in the clutch and gave it gas the engine would rev. just fine.

I may have confused the issue by stating the car was acting up when cold implying it ran fine when warm when in fact it ran so poorly at the start (again it STARTED and IDLED just fine) that I didn't drive it long enough to see if it continued when fully hot.

[ianc]

Alright, I think I see what you're saying. The switch never opened properly, so the DME thought the car was always idling. If that was the case, any attempt to rev the car would cause cutting out because the DME would stop fuel to the injectors thinking the car was idling.

I thought you could rev the car, but it only bucked when you put it in gear and tried to drive it?

ianc

[bill3667]

Originally posted by ianc
I thought you could rev the car, but it only bucked when you put it in gear and tried to drive it?

ianc

This is a correct statement. Understanding it in any more detail is beyond my knowledge base. I do know it runs now.

Edit: Woops, didn't mean to imply it started off bucking, it would buck (fuel cut out) 50 ft. or so down the road, it would start to drive ok but it wouldn't go very far. It didn't start bucking as soon as you put your foot on the throttle which tells me the the DME is not always looking at the switch input.
I hope I've cleared it up.

[ianc]

I don't get it either. I don't believe the DME makes any distinction as to whether the car is in gear or not, or whether the clutch is in or out.

Be interesting (and helpful) to find out from your mechanic what the issue was exactly,

ianc