I thought I finally had it..stumbling acceleration
 

After changing the AFM, TPS, DME, fuel injectors, vacuum lines, ignition system, fuel pressure regulator, fuel pump, fuel filter, strainer, air cleaner, oxygen sensor, ref sensor, fuel injector connectors, Oil cap seal, AOS seals and catalytic converter I still have a stumbling acceleration problem during normal acceleration.

On Friday I did another vacuum leak check and really looked deep into the guts of the motor. I noticed bubbles coming out of the hose where it fits onto the AOS. So I pulled the intake manifold to get at it and in the process, found 2 of the 4 intake manifold gaskets weren't in the proper orientation. The little notch in the gasket was not where the injector sprays into the head and so I was convinced that this, plus the AOS leak, was the final solution (poor choice of words, I know).

Today I put everything back together and it still stumbles while accelerating. WOT works fine and it idles just as it should. At this point, I'm simply out of answers.

I don't dare take the car to the dealership where they charge $135 an hour and don't really have a 944 tech in their shop. So now I'm wondering what the next step will be. :rolleyes:

Plug wires? or were they included in the ignition system? Same with the coil.

Very strange indeed.
It would appear that you have covered your bases. Fuel, and ignition, look to be the most probable culprits. Coil or fuel pump is my guess, and bare in mind that a new component may be faulty right out of the box... but unlikely to be malfunctioning in the same manner as the replaced part.
I see you said you replaced ignition system, so I assume you did the coil also...

It would be nice to see what your fuel pressure gauge does while you are experiencing this "stumbling". Obviously the hose is too short, so you might have to get creative... maybe put a camera on it?

Also, can you describe the stumbling?

I noticed you still have the original fuel lines... maybe an insurance claim would fix it. ;)

There are different types of hesitations that could help narrow it to either electrical, fuel or vacuum, unless it is worse (i.e. the motor). Give this a try:

https://www.google.com/url?sa=t&rct=j&q=&esrc=s&source=web&cd=9&cad=rja&u act=8&ved=0CFgQFjAI&url=http%3A%2F%2Fwww.twistedan des.com%2Fforo%2Fattachment.php%3Fattachmentid%3D6 6395%26d%3D1250882818&ei=IjGaU7XSLZGWyATu2IGYBQ&us g=AFQjCNG4HO1PgpZGJunCcN68kicBtT0H3w&bvm=bv.689119 36,d.aWw

replaced the wires with bosch/beru and coil, both used and didn't change the symptoms.

I like how you think:) Actually, I replaced the fuel pump and FPR and did all the recommended fuel system checks before and after and still no change other than new parts. I replaced the coil with a used one and tested the continuity of the plug wires before I installed them with a new cap, rotor and plugs (properly gapped). I pulled the NGK plugs the other day and they read perfect. Just to be sure, I replaced them with Bosch Copper + and still no change. The TPS is new and you can hear the click as you move the throttle off full closed position. All the parts I changed from known good cars had no effect.

I'm wondering if it would make any sense to treat the new TPS as if it was used and test the coil and other components that were replaced to verify their function, but each time I changed something, there was no change.

The stumbling is simply that: the engine falters noticeably while under normal acceleration. It doesn't feel like a classic ignition miss and the engine idles smooth and at WOT it rocks. I suppose I could put an extra length of fuel line in the line that goes to the fuel pressure gauge I have connected to the fuel rail and put it up under the windshield wiper to see if it changes in time with the hesitations.

It's just so strange... your engine sounds like it has so many new or rebuilt parts, that it should run better than most of the other ones out there......

Something when stressed, or overheated, just might start malfunctioning. But obviously you can't relentlessly use the shotgun approach and trow part after part at it.

Can you recreate the stumble during a no load condition, while the car is stationary? If not, and this is a shot in the dark, maybe it's a drivetrain or clutch issue?

Hopefully someone will chime in with a solution.

Weird
 

Sounds like you have covered about everything. It sounds most like a vacuum leak.

Decades ago I had a 356B with one spark plug wire laying against another plug wire. The two would induce a current in each other out of sync causing the engine to run rough. The fix was simple, separate the wires far enough apart that they didn't induce a mild misfire in the other cylinder out of sync. Your wires have separation from each other I hope? Good luck finding your problem it will be interesting hearing what it is.

Have you tuned up your AFM? Towards the end of this there is an AFM cleaning and repair procedure. http://www.clarks-garage.com/pdf-manual/elect-22.pdf

I did tune up the AFM and actually replaced it with another just to be sure and it came out of a car with no issues. Still no change and I tuned it up as well. Very challenging problem.

Interesting that you would mention this as, in the interest of being neat, all my wires are running parallel to each other using the wire looms and never considered this as a possibility. Since the coil I replaced the original one with was used as well, I've ordered a new Bosch unit and when it comes in early next week, I'll check into the wire routing as well.

I have a new clutch and the transaxle was pulled at the same time for servicing. When the hesitation occurs, the RPM do not fluctuate so I don't see the drive train as an issue, especially since I can go to WOT and it really moves, and without a hesitation. It doesn't do it without a load. Thanks for the thought.

How does the car run during warmup? Does it miss or stumble for a few minutes? Are the clamps tight on the ICV and AOS lines? Are the lines new as old ones could be cracked and leaking vacuum? What about the icv line clamp down under the intake near the dipstick tube.

If not these lines what about the vacuum lines on the left side of the engine compartment for the fuel vapor recovery, climate control, and heater control valve, etc? As a test cut off the end of the hose and reattach and also make sure all of the clamps are there and tight. There is also a vapor canister in the LF wheel well behind the plastic fender. Also try spraying starting fluid around the intake gaskets and vacuum lines to see if the engine surges because of a vacuum leak.

I am guessing vacuum as if it were electrical or fuel it would get worse under full throttle.

Actually, the car stumbles somewhat whether cold or up to normal operating temp. I just finished pulling off the intake manifold and tightened up the line that was loose on the top of the AOS and checking all the other connections. New intake manifold gaskets and AOS seals had been replaced last year. New silicone vacuum lines and still no change.

There is also a vapor canister in the LF wheel well behind the plastic fender from DJNOLAN.

I am guessing vacuum as if it were electrical or fuel it would get worse under full throttle.[/QUOTE]

I looked at my car and I see that there is a vacuum control input line from the throttle body into the shutoff valve (H on the diagram) for the canister. I'm wondering if I clamp that line off, It will isolate that part of the system so as to eliminate it from consideration. I assume that valve is normally in the "off" state, right?

Check your valve springs. A broken one will cause a miss under acceleration.

Holy Cow!!
 

I decided to follow the quick and easy recommendations of SYSGEN and KONAKAT (on that other forum) and disconnect the O2 sensor. It is a brand new Bosch 3 wire unit and don't know what it's disconnect might be masking, but IT WORKED!! The car runs like it should in all RPM ranges, regardless of the throttle application.

The only concern I have now is that the idle cycles +- 200 rpm. I'm 100% sure that there are no more vacuum leaks so whatever it is, I can live with it, just to get the car running like it should. Now on to recovering the front seats.

Thanks to all of you whom offered your inputs. I appreciate your involvement and insights. I'm sure that any one of the ideas could have caused the symptoms I was experiencing. SmileWavy

Stock chips and DME? FQS switch in stock #0 position?

Check the wiring for O2-sensor. It's a shield-coax cable to the DME. I've seen many cars where people used vampire-taps to extract the O2-signal for an AFR gauge. However, this shorts out the O2-sensor signal by connecting the centre of the coax-cable to the shield.

At the DME, check for continuity between the O2-sensor's centre signal-wire and its shielding. Shouldn't be any. Also wiggle the O2-sensor cable to verify there isn't a short somewhere along the way.

Stock DME and chip, new O2 sensor and wiring inputs checked to connectors and shield. Tomorrow, another shot at possible vacuum leak (3rd time but this time with chemicals). No extraneous or peripheral devices are wired into the car.

This is typically a sign of intake-leaks and/or misadjusted throttle-plate, idle-screw and idle-stabilizer. Follow the manual's procedure for disabling the idle-stabilizer and setting the idle-screw. With the idle-stabilizer disabled, setting correct idle-speed on a warmed-up car will have it not contribute any extra air to idle-speed. No catch-up game with overshooting and undershooting.

You may also have a sticking ICV. A little carb or TB or MAF cleaner down the ICV hose may free it.

You're right on about the settings. I had gone through all that before I disconnected the O2 sensor. I'll have to go through it again after I do a re-check of the vacuum for leaks again. While I had the intake manifold off last week I cleaned the ISV and rechecked the lines for leaks.

What are the pin assignments on the O2 sensor round plug (3 wire)? I've checked everywhere and can't find a diagram for which pin has input, shield, heater wire.

O2-sensor signal wire goes to pin# 24
O2-sensor coax-shielding goes to pin# 23 (also shared with shielding from speed-sensor).

There should be no continuity between these two pins. Have an assistant wiggle the O2-sensor wires in the engine-compartment while you watch the voltmeter. Also disconnect and connect the O2-sensor, shouldn't have any continuity.

While there, you'll want to open up the wiring-connector and solder a wire to the back of the #24 terminal-pin to extract an O2-sensor signal for monitoring. That's the best location to get it. Many people run a wire out to the engine-compartment and hook it up to the O2-sensor connector just before the it connects to the shielded harness wire. But then you've got an unshielded cable running parallel to the shielded cable and you won't be seeing the exact signal that the DME's getting.

With a fully-warmed up engine, you should see the O2-sensor voltage flip back and forth across 0.5v about 4-5x per second: 0.8->0.5v->0.2->0.5v->0.8 etc. It should be symmetric on both sides of 0.5v. If it's leaning heavier towards one side or the other, then you may have a problem with O2-sensor and/or wiring. Use pin#28 for sensor-signal ground (not power ground or body).

1. Also check the mechanical position of the throttle-butterfly. There's a picture in the manual on it's positioning, something about just barely after a tiny hole or something like that.

2. And check the throttle-butterfly axle for leaks, that can actually be significant enough to mess with idle-speed and the ISV. There's a write-up somewhere along with list of O-ring parts needed to re-seal.

3. check the O-ring under the throttle-bypass screw (idle-speed adjustment). If O-ring isn't fully sealing, that too can be a source of leaks.

Sounds like all the stuff that I've done over the last year. I replaced the O rings in the throttle body shaft when I did my initial vacuum checks. Bubbles came out of the TB under the Cam. Also checked the idle speed adjustment O ring...Good.

I'm not sure what you're referring to on the butterfly positioning.

The angle that the throttle-plate rests at when you release the throttle is adjustable. It should be behind a small hole on the edge of the throttle-body. That hole sends a vacuum signal to the EVAP system. If the throttle-plate isn't set properly, you could be sucking in extra fuel from the charcoal-canister. Although this would be more troublesome at idle than under acceleration.

The stumbling issue goes away when you disconnect the O2-sensor. Assuming that the replacement O2-sensor is working properly, then we have to assume that something is wrong with the O2-sensor wiring. If you've verified that the O2-sensor signal-wire is not shorted to the shielding braid, then we should test the O2-sensor signal at the O2-sensor plug itself. Then test it again at the DME terminals to make sure it's the same signal.

If this is a 944na, we should verify that the full-throttle switch on TPS is working and that terminal activates properly at the DME terminal. If a 951, should verify that full-throttle signal is coming from KLR.