PhatPhlatSix

Done that When I put the cat bypass on, the old O2 sensor was a bit reluctant to come off the cat, so I forked out for a new one.... Damn I spent a lot of money on this

pncarrerars

Hi PhatPhlatSix, can't believe that I missed this thread. FYI & to add weight to your theory that this is a standard 'feature' of the 964 I also have a car (std 964RS) with exactly the same symptoms. I have been trying to sort the problem on & off ever since I bought the car in 1998 & had reached the conclusion that it was an enrichment problem but never followed it up so congratulations to you for seeing it through to the end.

Pete

PhatPhlatSix

OMG, we have a believer in our midst!! Praised be the lawd of high octane gasoline..!!

PhatPhlatSix

I would like to help any Rennlisters suffering this problem, and also recoup some of the cost I've sunk into fixing this problem. So, if anybody wants a modified chip for a modest cost, please PM me.

dlpalumbo

I've been dogged by this same issue (throttle bog at low RPM). However, I've thought it was related to another common issue: idle bounce. When I lift the throttle, my engine sounds like it cuts off then refires as the RPM passes the idle setting, then pops above idle before settling back down. Thus the bounce. The recommended way to fix this issue is to do a fuel adaptation - every time you disconnect the battery.

I thought this can't be right, just like the throttle bog doesn't seem right.

I read a post where someone tried to adjust the ISV to eliminate this issue. I tried the fix. This is what I learned.

The first photo is of my original ISV. Note the small opening. My thinking was that this was a rough idle set point that the ECU would adjust once the RPM dropped near idle.

The second photo shows the hex bolt added to adjust this set point. My thinking was that by increasing the opening I would raise the lowest RPM the engine would drop to, thereby eliminating the bounce.

Didn't work. No matter how far I backed out the adjustment, the idle would still bounce. After some further digging I realized the direction the ISV was 'opening' was initially closing the gap. Note the arrow in the first photo. As the cylinder is rotated, the initial opening completely closes before it begins to open again at the top of the inlet. The ECU will open a gap at the top of the inlet to increase the idle.

This made me think at first that the ISV might be wired backward, but when I looked at it again this morning, I realized that the mechanism cannot rotate upwards. It's against the stop.

So if the ECU's guess at the idle point is wrong, that is not open enough, it could completely shut off the air which would, I think, shut off the fuel, producing a lean condition. The fuel adaptation procedure would cause the ECU to remember the current ISV setting as a better first guess.

So what is the purpose of the opening if not an initial idle set point? I learned today that if I cranked down on the set screw, I can begin to open a gap at the top of the inlet (may actually be the outlet) which would mechanically set a lowest possible idle point and might eliminate the bounce and maybe some of the lean conditions.

Could a batch of ISVs been set incorrectly?

PhatPhlatSix

The ISV draws air through the air-flow meter - it merely bypasses the throttle butterfly, so the AirFuelRatio should remain constant/consistent regardless of how much assist the ISV is giving.

alexjc4

I wonder of its a symptom of a worn AFM? When cold the idle is maintained by the enrichment and cold startup system, but once hot the AFM isn't metering the air accurately enough around idle (were the wiper tracks get worn) to manage without the addition cold accel encrichment.

dlpalumbo

I actually opened the AFM to take a look, Didn't look worn, but didn't take measurements either.

Seems to me that the car would not have had these idiosyncrasies when new. I've learned to live with it but I'm always on the lookout for a way to fix it.

Anybody know if the requirement to do fuel adaptation any time the battery was disconnected was an early requirement, or something that evolved as the cars aged?

Goughary

If the afm were the issue, anyone that has replaced the afm with an aftermarket maf or similar wouldn't have this issue.

Mine is stock, and I have this issue as well. Even with the Steve Wong chip. Car runs perfectly in every regard except the throttle response has always not been good.

bucker

Thanks PhatPhlatSix for the work you've done to resolve this issue.

Fitted the new chip today, easy after watching Motronic ECU's being taken apart on YouTube. Only done a short run but immediately notice smoother running a tick over, throttle blips are easier tho my technique is poor. Massive improvement around 5000rpm, the pull keeps coming all the way to the red line.

Overall, very pleased.

Lorenfb

Thought the "fix" was only for poor low RPM revving, i.e. "Massive improvement around 5000rpm"?

PhatPhlatSix

When I was doing the investigation, I tried a variety of chips. Having installed my AFR meter I checked out low end performance, and then also the top-end performance out of interest. I noticed the stock chip has an anomaly at around 5000rpm. My AFR confirms this. No names mentioned, but there are after-market 'power' chips out there with this anomaly in them, i.e. the WO Throttle injection map has just been lifted by a factor along the whole board, without targetting any individual problem areas. So, to cut a long story short, I produced and tested an improved version which I offered in addition to the throttle-blip fix.

Lorenfb

Would expect your efforts will now be incorporated (copied) and promoted by "after-market 'power' chips" without
providing credit where it's do as is usually the case.

alexjc4

Well who'd of thunk it. I had a drive out tonight and I totally also have the "easy to blip cold but hard to blip warm" syndrome!

Steve W

On 964s, lack of low rpm response is usually indicative of a problem with one or more of the following portions of the motor:

• One side of the twin plug ignition system not working or firing properly. When one side has failed the car will be 20 hp off across the board, and have poor throttle response. Common causes are crossed wires, failed coils, broken distributor belt, or leaky plug wires. To test, with car off, disconnect one of the ignition coil modules, then start and drive the car. It should run smoothly and normally but a bit soft. Turn car off, reconnect the and disconnect the ignition module for the other coil. Repeat drive test. If one side doesn't start, or sputters, or runs badly, there is a fault in the ignition system that needs to be traced.

• Failed distributor hall sensor. When that fails the DME also runs in a detuned mode with HP severely off. A Porsche scanner such as a PST2 or PIWIS will tell you if the sensor has failed

• Fuel quality switch in DME not in stock CCW position. It is only the early 964 units that have this switch.

• Worn carbon track in the air flow meter (AFM). When the meters get over 100k mile of use, there is usually noticeable wear on the first 25% of the track which causes the voltage out signal to be lower than it should be or complete signal dropouts, causing lean surges and miscalculation of load for the part throttle ignition timing. Repositioning the wiper tip to sweep on a new arc restores the signal like new, and provides the correct acceleration fuel enrichment and part throttle ignition timing for throttle response and drivability.

The following, although not related to low rpm throttle response, are also common failures on 964s:

• Resonance flap failure. The intake resonance flap goes from closed to open at 5500 rpm, shortening the length of the intake runners, moving the resonance tuning of the intake to a higher rpm. When the flap fails, it does not open, HP peaks at 5500 rpm and falls sharply thereafter, to loosing as much as 30 hp by 6500 rpm. When your dyno chart peaks at 5500 rpm, and AFRs go very rich thereafter, this is your first clue. The failure can be attributed to a failed vacuum solenoid, bad vacuum actuator, or sticky resonance flap.

• WOT failure. With someone pressing down fully on the gas pedal, the throttle butterfly at the throttle butterfly should open fully and rotate a full 90 degrees.

• WOT switch failure. There is a plastic cam on the side of the throttle body's pivot shaft that can disintegrate and fail to activate the wide open throttle switch, which tells the DME to ignore the O2 sensor and switch over to the full throttle maps for max power. An multimeter should also be used to test the switch to confirm that it makes a consistent on/off connection when it's activated.

With all of the above functioning properly, there usually should be no need to alter the sensors or retweak programming. Doing so is only masking and band-aiding the actual problem which should be addressed first. Properly sorted, with everything properly working, a 964 motor drives pretty spectacular with excellent throttle response.