993 engine bogging down at 4K

[Jack Olsen]

This is a mystery I haven't been able to solve.

I have a 1995 (OBD1) 993 motor in a 1972 chassis, and after over a decade of trouble-free performance, I'm getting a sudden bogging down (loss of power) above 4k when accelerating under load.

Prior to this, I had a no-start issue that was solved by reattaching a borken 12v line in the under-the-seat 14-pin connector. I also had an idle issue solved by replacing the MAF sensor. But fixing those two things revealed the 4K bogging issue.

Here's a video example of what I'm talking about: https://youtu.be/MN1dlEUd5_4

I do not have a diagnostic port on it, but I'm working on that. I also have someone who's generously offered a 993 ECU I can swap in, although that won't be for a couple of weeks. So those pieces are pending.

So far, I have swapped out the DME relay, the ignition ignitor, the MAF sensor (two different replacements), the coils, the fuel filter, fuel pump, and fuel pressure regulator. I swapped in a generic O2 sensor, but the car has always run well with the 02 sensor connected or disconnected, so I don't think that's the issue. The dual distributor belt is fine and the rotors are oriented the way they're supposed to be.

Any advice appreciated.

[al lkosmal]

Jack,
Can you datalog your AFRs while driving? I was out road tuning today (2.7RS with EFI/ITBs/Megasquirt) and had a similar condition....at 4k-ish the engine would stumble/miss....above and below 4K it ran fine......It felt like an ignition miss to me and i chased that for awhile, but my datalog revealed that I as going slightly lean during accel....from 3500-4500 rpms. Added a bit more fuel to that area and the issue is gone. I know that you may not be able to modify the fuel table, but logging the AFRs my reveal what is happening ....not necessarily why, but could eliminate that as a cause.

[Jack Olsen]

Thanks. It stumbles at every point above 4k, as far as I can tell. It's got a custom (Steve Wong) chip for its A/F map, which -- again -- has worked fine for 11 or 12 years. Still, it could be that. I've got no way to monitor it right now. And it's hard to think of a reason why it would change.

[OSC911]

Hi Jack

Have you confirmed with a timing light that it is firing on the upper and lower plugs. I routinely put a timing light on the upper and then the lower coil leads/wires and see if my light flashes. My ignition igniter failed once and I was only getting ignition to the upper plugs. I know you have replaced the igniter but worth checking as mine had similar symptoms as yours.

Cheers
Todd

[al lkosmal]

Jack,
Yes, it's not likely that the chip/map has changed, but maybe the fuel pump/pressure has changed and that may also show up as a lean condition/stumble, under load.....

regards,
al

[blue911rsr]

Jack
If you have a wiring diagram I would pin out the wires
From maf to ecu . Have seen issues with that on cars
That had engine and wiring swaps like that .
Good luck

[Jack Olsen]

Originally Posted by OSC911

Have you confirmed with a timing light that it is firing on the upper and lower plugs.

Thanks. I don't have a timing light, but the car runs with either coil lead disconnected.

Originally Posted by al lkosmal

Yes, it's not likely that the chip/map has changed, but maybe the fuel pump/pressure has changed and that may also show up as a lean condition/stumble, under load.....

If I don't solve it, I'll have to check fuel pressure at different RPMs and an A/F ratio sensor after that.

[Steve W]

Jack, does it matter if there's load or no load on the engine when it stumbles after 4k? i.e. full throttle vs just cruising or in neutral. If it happens no matter the load condition, maybe the reference sensor is going bad. Otherwise if it's load dependent, then probably not that.

Was the distributor recently pulled out and reinstalled in the motor? I've seen many times where the rotors were installed incorrectly a tooth off in relation to TDC and the motor would cut out or studder hard after 3500-4k, because the rotor is not contacting the corresponding cylinder in the distributor cap when the DME calls for the ignition to fire.

[Jack Olsen]

Thanks, Steve. With no load, there's no stumble. In first gear, even, it will pull all the way to 6K. But in 2nd, 3rd, 4th, or 5th, it hits that point (around 4.1k) and stumbles from there on up.

There might also be a slight hesitation on initial throttle application, like when the throttle goes down on a rolling stop in 2nd. But I'm not sure if that's just in my head.

I haven't had the distributor out. When I checked the orientation of the two rotor arms, they happened to both be at the TDC bump on the side of the casting, so I think they're correct.

[gduke2010]

I have the same engine and had an issue a while back. The engine would miss fire and took it to a small Porsche/VW shop. They couldn't find the problem. My car would run fine at lower rpm's and miss accelerating. It sounds like a Subaru WRX. But, the problem was intermittent.

Anyway, someone recommended taking to the Porsche dealership where they immediately, found the car needed a new wiring harness. Also, heard this was common problem. This was over 10 years ago and the new harness was about $1000 installed.

Might be something different with Black Beauty. I looked at your post and seen nobody mentioned the wiring harness.

[mikedsilva]

Originally Posted by gduke2010

I have the same engine and had an issue a while back. The engine would miss fire and took it to a small Porsche/VW shop. They couldn't find the problem. My car would run fine at lower rpm's and miss accelerating. It sounds like a Subaru WRX. But, the problem was intermittent.

Anyway, someone recommended taking to the Porsche dealership where they immediately, found the car needed a new wiring harness. Also, heard this was common problem. This was over 10 years ago and the new harness was about $1000 installed.

Might be something different with Black Beauty. I looked at your post and seen nobody mentioned the wiring harness.

Yes, a fellow I know worked on my 964 and also told me of factory recalls to replace the complete harness. Not sure what the actual suspected error was though and it's possible a lot of cars did not get that recall completed.

[shahram]

Jack, did you replace the MAF with a new factory MAF sensor?

[OSC911]

Not something simple like the ignition wires breaking down under load - when we’re they last replaced?

[r_towle]

First I would monitor fuel pressure and change out your fuel filter. Gas today is clogging things up after sitting.

Second, timing, timing advance method/mechanism

Third, power and grounds…full check to all ECU, alternator and ignition switch

[Zeke]

Wiring sounds culpable but I sure wouldn't go down that road. I'm thinking a dyno session is in order from somewhere Steve Wong recommends as they will have all the external sensors to add to the data.

[Burgundy T]

Jack, I had very much the same symptoms with a 95 993 transplanted into my carerra. Turned out to be a bad connection between the harness and the head temp sensor.

[Jack Olsen]

Originally Posted by shahram

Jack, did you replace the MAF with a new factory MAF sensor?

The Bosch wasn't available, so I got the cheaper brand. But then I got another (cheap, but different) one. If the first one had the problem of failing at higher velocities, it seems kind of a stretch that the second one would do the same thing. But it's not outside the realm of what's possible.

Originally Posted by Burgundy T

Jack, I had very much the same symptoms with a 95 993 transplanted into my carerra. Turned out to be a bad connection between the harness and the head temp sensor.

Cylinder head temperature sensor is also an easy/inexpensive thing to try. I'll order one.

[3rd_gear_Ted]

When in doubt unplug the ECU and reconnect it to reestablish a fresh mechanical bond and to let the bad juju out of the ECU.
Same with other connectors if possible.
I have a #8 ground wire from my ECU enclosure to the body on installed studs.

[Jack Olsen]

I'm still trying to isolate this problem. And I'm interested in any suggestions of where I could look next.

So far, I've replaced/swapped:

Ignition igniter
MAF sensor (x2)
DME relay
Fuel filter
Fuel pump
Fuel pressure regulator
Coils
Entire DME Box/Control Unit
Cylinder head temp sensor

I've got a crankshaft position sensor on the way, although I don't think my easy startup and idle matches with this part going bad. I'm doubtful that it's as simple as dirty injectors, but I could run some Techron through it.

The bogging is right at about 4,200 rpm, under load, and is fairly consistent. Sometimes it will then plateau at 5K and not go higher.

I've run with the O2 sensor connected and disconnected with no change, so I don't think it's that.

[shahram]

Can it be that the ECU going ti kinda limp mode? Jack i think you need to make your ECU readable.