Hesitation on Acceleration and Idle Stalling

[rusnak]

Converting %CO to AFR: (by Scarceller, post #122 in this thread )idle bounce

%CO AFR
----------------
0.1 14.72< dme connected, warm O2 sensor*

0.2 14.54
0.3 14.42
0.4 14.34
0.5 14.28
0.6 14.23
0.7 14.21 < dme disconnected**
0.8 14.17
0.9 14.15
1.0 14.11
1.1 14.09
1.2 14.04
1.3 14.01
1.4 13.98
1.5 13.94
1.6 13.89
1.7 13.86
1.8 13.82
1.9 13.80
2.0 13.77
2.1 13.73
2.2 13.69
2.3 13.63
2.4 13.59
2.5 13.56
2.6 13.54
2.7 13.49
2.8 13.45
2.9 13.41
3.0 13.38
3.1 13.34
3.2 13.31
3.3 13.27
3.4 13.24
3.5 13.20
3.6 13.15
3.7 13.12
3.8 13.08
3.9 13.03
4.0 13.00
4.1 12.96
4.2 12.93
4.3 12.90
4.4 12.86
4.5 12.83
4.6 12.80
4.7 12.75
4.8 12.70
4.9 12.66
5.0 12.64
5.1 12.59
5.2 12.54
5.3 12.51
5.4 12.46
5.5 12.43
5.6 12.40
5.7 12.37
5.8 12.30
5.9 12.25
6.0 12.25
6.1 12.22
6.2 12.18
6.3 12.13
6.4 12.10
6.5 12.09
6.6 12.03
6.7 12.00

* Notes added by me

[bhoff]

A quick update. I just figured out that I think I have two problems. Like and idiot, I did not take the car for a drive after replacing the flywheel sensors the other day. I just started the car and noted that it would barely idle so assumed nothing had changed. Tonight I took it for a short drive and had no issue with the stall out and sudden jerk back to life. In fact, the power and acceleration difference was amazing. I think this issue had to to with the flywheel sensors only. These sensors must have been bad for some time and I did not know it. For what ever reason, these sensors would work better after the engine was hot. Idle is still poor with the backfires. Rusnak, I think my next step will be to test the O2 sensor with the information you provided. Thanks for all of the input here. I feel better that one problem appears to have been solved for now.

[bhoff]

I spoke too soon on the hesitation. It seems to be better but still some stalling on take off from a stop. I took a look at my vacuum hoses and found one that was off! It goes to a valve on the oil tank (PCV setup?) Looks like it had been knocked off putting the air filter cover on at some point. I thought I had solved my problem but putting it back had little effect. To me this would suggest I may have more vacuum leaks. Bought some vacuum hose and will be replacing what I can. I also ordered an O2 sensor and will install tomorrow.

A couple of questions;

1) after installing the new O2 sensor, I understand that the air flow meter will have to be adjusted for correct mixture. Not sure how to do this without anyway to monitor the exhaust. I am wondering how important it is to get this set right. Is this something I can get done in the near future instead of immediately after replacing the sensor?

2) I used some starter fluid to check for vacuum leaks. Inconclusive as to if there are any but will be replacing some of the smaller hoses. I read somewhere in this forum that removing the oil tank fill cap will affect the mixture and should change the engine rpm slightly. Taking my cap off has no effect that I can notice. Would this mean that I have other vacuum leaks?

Thanks guys for all of the input.

[rusnak]

Maybe. That's all one can say on an online forum, maybe.

In "Bosch Fuel Injection and Engine Management" by Charles Probst, the author describes a lean stop test in closed loop by removing the oil filler cap. The car should stumble, then recover. Same as removing the fuel pressure regulator vac hose.

If you car is running in open loop, or at startup, there will be no change.

You are shooting in the dark without a way to test your air fuel ratio.

When people adjust the Air Fuel Meter, they do so with the O2 sensor disconnected (open loop) in order to set the baseline. I think most guys tend to adjust it rich, and let the dme lean out the misture. Seems the 911 likes 14.2 AFR in open loop, which is a tad rich.

[rusnak]

I think you might be talking about the vac hose between the ambient air valve and the throttle body. Check the crankcase vent hoses. You will have to remove the air box and air flow meter to really get in there and feel with your hand. You might have a huge vacuum leak. That would make sense to me. Also make sure your vacuum line to the fuel pressure regulator is not leaking and is pulling vacuum. Do this before installing the new O2 sensor.

[86 911 Targa]

Originally Posted by bhoff

I spoke too soon on the hesitation. It seems to be better but still some stalling on take off from a stop. I took a look at my vacuum hoses and found one that was off! It goes to a valve on the oil tank (PCV setup?) Looks like it had been knocked off putting the air filter cover on at some point. I thought I had solved my problem but putting it back had little effect. To me this would suggest I may have more vacuum leaks. Bought some vacuum hose and will be replacing what I can. I also ordered an O2 sensor and will install tomorrow.

A couple of questions;

1) after installing the new O2 sensor, I understand that the air flow meter will have to be adjusted for correct mixture. Not sure how to do this without anyway to monitor the exhaust. I am wondering how important it is to get this set right. Is this something I can get done in the near future instead of immediately after replacing the sensor?

2) I used some starter fluid to check for vacuum leaks. Inconclusive as to if there are any but will be replacing some of the smaller hoses. I read somewhere in this forum that removing the oil tank fill cap will affect the mixture and should change the engine rpm slightly. Taking my cap off has no effect that I can notice. Would this mean that I have other vacuum leaks?

Thanks guys for all of the input.

If there is no change in RPM's when the oil filler cap is removed, I think you have a vaccum leak.

I had a similiar issue on our '86 a few years ago, and found a (self inflicted while replacing a vent hose) disconnected vaccum line from the throttle meter to the pressure regulators.

Good luck,

Gerry

[bhoff]

Well I took the AFM off and checked the vacuum hoses. I didn't know these hoses even existed behind the engine. Replaced the long one running to the valve by the oil tank as it was quite brittle and a fit loose. Replaced a short 2" hose that ran from one outlet on the body to another (not sure at all what this one was for). Several of the larger hoses were loose so tightened them. The teed hose running to the fuel regulator/dampener looked ok. On first start, the exhaust was blowing black smoke like I had never seen and stumbling severly. Shut it off and on the next start it ran pretty well only missing slightly. Took it for a drive and it ran well and did not stall during idle. Car sat for several hours and drove again cold. I could barely keep it running at idle. Missing and some back firing. I am not sure if it is running better or not. It seems to run better now when it is warn but the same or worse when it is cold. From what I am understanding here, it may be running worse in the open loop and more normal when it gets to the closed loop.

I will be installing the O2 sensor tomorrow. I am having my doubts that this will have any effect. Thinking I need to check the fuel pressure like rusnak recommended. New fuel pump but the regulator may be bad. I see there is a "fuel regulator" on one of the fuel pipes and a "fuel dampener" on the other. They look identical in the photos. Does anyone know if there is a difference?

I am amazed that after finding loose vacuum hoses and a hose that was completely off I am still having similar issues. One of the greatest difficulties I am having is trying to find some consistancy to my problems. Everything I have done to date could have possibly fixed it yet they all are having some effect but not all positive. Or maybe they are positive but I just cannot figure out how they are affecting the entire system. Brain is fried now so much I am repairing my car in my dreams at night!

[swhite7477]

Guys, I am having a terrible cold start driveability problem on my 83 SC. Sounds very similar, but my problem literally happened overnight after a 700 mile trip. The next morning, it started fine, but after 60 seconds the idle dropped to 500 rpm and it began running very rough, no smoke though. After 20 minutes it smooths out some, but still hesitates and does not climb the rpm range smoothly through the gears, but will operate at highway speeds once fully warmed, but when exiting the interstate and stopping at traffic light, it still returns to the 500 rpm rough idle.

I just had the car tuned(2000+) and serviced, but I am TDY 700 miles from home, cant take it back to them for 3 months. The thing that caught my attention here is that when I picked the car up from the tune-up, the tech said he unplugged my o2 sensor and plugged it, these older cars run better that way. This is a 3rd generation owned shop, I trust them, but am open to other opinions...Thanks

[bhoff]

Back at it again after taking a break. I posted some questions a couple of weeks ago and want to thank those that responded. I am still having problems with hesitation, stalling, rough running, and sometimes up and down rpm at idle when the engine is cold.

I changed the O2 sensor. At this point, all sensors have been replaced. Everything I have done so far has helped some, but I am still getting these issues.

I do have a new issue that I did not notice as much before. Under hard acceleration sometimes the engine stalls for a brief second and then comes back strong.

My question is that when I take off the vacuum hoses on the fuel pressure regulator or dampener, nothing happens. It does not matter if I take off the hose and plug it or leave it open, I do not notice any change in the engine. There seems to be good vacuum on the hoses and they were tight on the units so I do not think they are leaking. I also notice that nothing happens when I take off the oil tank cap. I have not been able to check the fuel pressure yet lacking a gauge. Does this sound like I still have some hidden vacuum leak or more likely a bad fuel regulator. I know I really need to get a hold of a fuel pressure gauge to check the regulator. Thanks for any help.

[bhoff]

I forgot to mention that after replacing the O2 sensor, the cold start did not change (still rough and stalls) but when it warms slightly, after about 2-4 minutes, it starts to smooth out more. I think this is when it goes into the closed loop cycle, which probably was not happening before due to the bad O2 sensor. The hesitation and stalling is substantially less but still there along with the hesitation on hard acceleration. Other than installing the O2 sensor, I have done nothing to adjust the mixture yet.

[ischmitz]

When you take the oil filler cap off and put your hand over the filler neck you should be able to feel the engine sucking air. If not something is wrong.

The fact that your idle tone doesn't change when you remove the oil filler cap points towards an intake leak: How about an old simple vacuum leak test: Use carb cleaner or starter fluid and spray a little into areas where you could have leaks: Intake manifold to head gasket, intake, hoses, etc. Once the idle changes a lot you found a leak.

Make sure you have a fire extinguisher nearby.

[AlfonsoR]

Subscribed...good info here.

[NoLift911]

Originally Posted by ischmitz

When you take the oil filler cap off and put your hand over the filler neck you should be able to feel the engine sucking air. If not something is wrong.

The fact that your idle tone doesn't change when you remove the oil filler cap points towards an intake leak: How about an old simple vacuum leak test: Use carb cleaner or starter fluid and spray a little into areas where you could have leaks: Intake manifold to head gasket, intake, hoses, etc. Once the idle changes a lot you found a leak.

Make sure you have a fire extinguisher nearby.

X2 - I know you torqued the intake manifold bolts but...what happens is the bolts become loose and it is possible for the gasket to get sucked in and torn or become missing altogether. It sounds like you have a large vacuum leak somewhere and the fact your idle does not change at all when removing oil cap or other vacuum hoses point to this.

[scarceller]

First, I did not read all the posts. But a engine shutdown then back to life sounds a lot like a bad ref or speed crank sensor. Does the tach drop to 0RPM like a rock when the shutdown happens? If so the DME is shutting down because it has not seen speed sensor pulses.

[bhoff]

Scarceller, I just replaced crank sensors. Also HTC and O2. I also sprayed starter fluid around the vacuum hoses and around the intake manifold. Will try it again. It feel like there is a pretty good vacuum suck on the hoses to the fuel regulator and fuel dampener. Don't have a vacuum gauge to actually check though.

[bhoff]

Scarceller, yes the tach drops like a rock and I can hear the air sucking through the wide open throttle. Comes back to life in about 0.5-1 second with a sudden violent thrust.

[scarceller]

At idle if you open the oil tank cap you should hear a difference in idle as opening the cap induces an air leak into the rubber 90deg boot that sits between the AFM and the TB. The oil tank breather pipe goes directly to the backside of this 90deg boot.

Also pinch off the pipe that goes to the power brake booster, this pipe is near the fuel filter on the left side. Clamp it off with vise grips or remove it and cap it at the vacuum side. If this pipe gets a leak it's hard to find since it runs to the front of the car.

Lastly, if you suspect a air leak and can't find it have the system smoke tested. Ask around to see who has a smoke machine in your area.

But I think you may have 2 issues here: one a possible air leak, the other is that you seem to indicate the DME simply resets (reboots) and this is indicative of RPM Tach dropping like a rock when engine cuts out. You already mentioned new FlyWheel sensors so it most likely is not that. The only other thing to try is to take your DME and put it in another car to see if the problem moves to the test car. DO NOT EVER, EVER take a DME from a good car and put it in a questionable car! Always move the DME in question to a known good car. WHY? because often DME failures are because of shorted sensors/components such as Coil or Fuel Injector and a shorted component will destroy the drive transistor in the DME!

You may have a faulty DME with a bad solder joint.

Originally Posted by bhoff

Scarceller, I just replaced crank sensors. Also HTC and O2. I also sprayed starter fluid around the vacuum hoses and around the intake manifold. Will try it again. It feel like there is a pretty good vacuum suck on the hoses to the fuel regulator and fuel dampener. Don't have a vacuum gauge to actually check though.

[rusnak]

Originally Posted by scarceller

Also pinch off the pipe that goes to the power brake booster, this pipe is near the fuel filter on the left side. Clamp it off with vise grips or remove it and cap it at the vacuum side. If this pipe gets a leak it's hard to find since it runs to the front of the car.

^ great tip. Also cruise control. He may have several vacuum leaks.

[bhoff]

Scarcellar, when I look at the large hose coming off of the oil tank to the rubber boot, I see that the boot is above the throttle body so there can be no vacuum. To have a vacuum, it would need to be below the throttle body. The smaller hose that goes to the valve shown in the photo should have a vacuum because it attached below the throttle plate. I do not know what this hose does but it appears to be attached to a PCV valve? When I take off the small control vacuum hose on the top, there is no vacuum on this hose. I do not know if and when this valve is suppose to be open or when there should be a vacuum on the control line. I cannot detect any vacuum when I place my hand over the oil filler and like I explained, I cannot see how there could be a vacuum here virtually being open to outside air pressure by being attached above the throttle body.

As to the DME, I did swap out a known good one last summer for a couple of weeks (prior to changing the O2 and flywheel sensors). I also did a visual inspection of the solder points on the inside recently.

[bhoff]

Tried to post the image but it would not come through. Here is a link that should work

http://img230.imagevenue.com/img.php?image=95814_OilTankHoses_122_337lo.jpg

Does anyone know what the trick to posting an image here?