BrianRMV

I've posted about my 1986 951 poor fuel economy (10 mpg) issue previously, but thought it would make sense to post under a new topic. I have a new mechanic working on my car. He seems very knowledgeable about the car, which is a lucky find in my area. He says the ignition timing is not advancing, but has not located the cause yet. Has anyone dealt with this issue? Any known causes would be appreciated.

Jfrahm

Maybe a bad knock sensor, that can cause a lot of timing pullout. Try disconnecting it.

odonnell

The TPS could also be broken and telling the DME to stay in a 2D map, where timing values have little variance.

BrianRMV

The TPS is new and I believe functional, although I have wondered if the harness is faulty. I read about faulty knock sensor symptoms and am really interested in testing that theory out. Thanks for the suggestion. I've done a lot of research but never had anyone suggest that before.

Tom M'Guinn

How did he conclude the timing is not advancing? Not saying he's wrong, but he wouldn't be the first to incorrectly believe that on these cars.

BrianRMV

Honestly, I don't know how he tested it. He is the shop owner, and was actually recommended by another shop because he has very good knowledge about 951s. He seems like a very thorough, knowledgeable mechanic. I have had the opposite experience too. I have spent a lot of money with mechanics who I think were honest, but really did not know these cars well.

Perry 951

Is this the same car WPO was posting about in this thread: https://rennlist.com/forums/924-931-...-t-change.html
Several of us asked a plethora of questions and never really got any concrete answers.

BrianRMV

No. I read that thread and wondered if he ever found the cause of his problem. If I find a fix for something I've posted about I'll let people know. I really appreciate any suggestions I receive, and hope someone else will benefit from my experience.

Perry 951

Since yours is a different car, can you respond with what issues/concerns you are having? What kind of diagnostics were performed and what were the results? What kinds of repairs have been attempted?

Tom M'Guinn

In your old thread, you described a bouncing idle and a host of other issues -- most of which would not be explained by ignition timing. Traditional distributors have mechanical advance that can get stuck or break and cause "no timing advance" --so that phrase is in most old-school mechanics' vocabulary. But that's not really possible on the 944, at least in the traditional sense. Apparent timing issues in the shop (where loads are very low) may help point to the problem(s), but it's a symptom and not a cause of whatever is wrong. If your mechanic is struggling, I'd be inclined to find someone who focuses on Porsche's, with as much 944 experience as possible. Seems like Seattle must have some good 944 guys...

Perry 951

I just found and read your old thread. Like Tom said, you posted about several concerns that may or may not be related. What I didn't read was when/how this began. Did it all of a sudden start idle hunting and running rich?

Lots of things have been checked or replaced.... but no mention of the fuel quality switch. Do you have any change if you try any other positions? If its in position 7 rather than position 0, you're adding 6% fuel and -3 degrees of timing. While cold, that is enough enrichment to make it run like crap. The Weltmeister chips will also have more aggressive maps that could add even more fuel. It's been 20 something years since I had a set in my car but I recall them being pretty rich in comparison to stock.

Perry 951

BrianRMV

Tom - Do you think any analysis from a dyno test would be useful? It's surprisingly difficult to find a mechanic for the 951s around here. I thought I found a good mechanic in Seattle a while back, but he had no idea how to diagnose this issue. I think the mechanic looking at it now is very qualified. One way or the other, I'll know fairly soon. I rarely see these cars around here anymore. I think most of the mechanics who worked on them have retired.

Perry - I bought the care around 15 years ago. For the first 13 years +- it got roughly 15 mpg. I just assumed 15 mpg was "normal" and never looked into what the fuel economy should be. A year or two ago the mileage started to drop off. That's when I looked into what other people are getting and realized I had a fuel consumption problem. The high idle issue started when I had the Rogue A-Tune installed. I'm fairly certain that the A-Tune was installed correctly, and it is not part of the problem. The Seattle mechanic I mentioned above fixed the high idle problem. I'm guessing that someone lost the old idle set screw, and replace it with whatever screw they found that fit the hole. Now it idles really rough when cold, but is perfect when warmed up. I have replaced the ICV, so I don't think it is faulty. The wire harness is a possible culprit. This is the only 951 I've ever driven so I don't have anything to compare it to, but when I get into higher rpm's it seems to hit a wall, if that makes sense. In case I did not mention this in my previous post, up until a couple of years ago it would max out at 1.4 bar on the boost gauge. Someone suggested that the cycling valve was probably shot, and installing a manual boost controller was an acceptable, reasonably cheap way to go. I have the manual boost control limited to 1.8 bar. I'm beginning to wonder whether or not the cycling valve and/or low boost numbers are symptomatic of my problem. Maybe this is another indication that the knock sensor is bad.

Thanks again for your input.

Perry 951

There are a few bits of info that could point us in some kind of a direction.

1. Assuming the 15MPG figure is in a good mix of highway and city driving (not a ton of stop and go), it's low. 10MPG is definitely low. I can get a reliable 30MPG highway in mine and that's with some big injectors, MAF, K27/8, and a light foot. In town is closer to 18-20MPG. All bets are off when I'm heavy footed.

2. 1.4 BAR on the factory gauge (6PSI) indicates to me that the KLR is likely sensing something and limiting boost. Bypassing the cycling valve to get 1.8 BAR simply masked the boost limiting function of the KLR. If the KLR detects knock (or another major fault), it will try and pull 3 degrees of timing to resolve. If no resolution, it will pull 6 degrees and limit boost to 1.4 BAR. I think it stays in this default mode for the key cycle. Has anyone pulled blink codes? If none are active, do you get codes by unplugging the TPS?

3. It's always been this way an no repairs/mods have resolved it. Your original tune (Weltmeister), or now the Rouge, could be overfueling and rearding timing due to the position of the FQS. I would check that ASAP.

4. The poor cold idle is typical when overly rich. When at operating temperatures, the rich condition does not affect idle as much.

5. With all this overfueling, it's possible the catalyst has some damage causing additional backpressure. This will not allow the cylinders to evacuate as they should and will cause a rich condition in and of itself. I would drop the downpipe from the turbo and see if the cold idle improves any.

In summary, I would check the FQS position to be sure it is set at the 0 point. I would then drive the vehicle and test for blink codes - if none found, unplug TPS and see if they set. If they do set, drop the exhaust and see how it goes.

BrianRMV

Thanks Perry. I'll check your suggestions out and let you know what I find.