Flat power spot in 3rd gear

[rbuswell]

1987 924S project car, manual gearbox, USA, mileage unknown but indicates about 51k miles (odometer failed who-knows-when but now repaired). PO estimated about 175k miles.

Some new OEM or better parts ... plugs, wires, cap & rotor, fuel filter, injectors rebuilt by RC Injectors, new timing belts, new K&N air filter, rebuilt alternator (on its last leg according to rebuild shop). Muffler's a little more flow-through but nothing radical. All things I would have done anyway given their condition. Use mid-grade fuel which is 87 octane in Colorado. CAT was replaced just before I bought the car and it passed Colorado emissions but not by much.


Things that could be a factor: Bosch fuel pump appears to be original, have not changed out fuel regulator, DME relay, or other control relays. Most of my driving so far has been near my home at 6,200 elevation.

Symptom: Starts easily, idles nicely, has good power from a standing start but goes flat mostly in high second gear and third gear about 2k RPM and up, settles back out at cruise (fourth and fifth). It acts like it's missing but you can't really hear that it is. It did the same thing before I replaced any parts. Gets a little better after the car has warmed up and been driven a while. All other vitals signs are good (temp and oil pressure, etc.). Seems to get pretty good gas mileage but don't really know since the odometer was just fixed.

[jjeffries]

We both have 82 SC coupes and 87 924S's. Different colors, though.

You'll need someone with more depth than me to give you the best advice, but I'd offer that the FP Relay is a car runs/car doesn't run component. Did the P.O. also operate the car at 6200' elevation? Do you have a fuel pressure gauge you could have hooked up while driving to see if pressure falls at the same time as the stumble you're encountering? That might be a good way to ascertain if this is fuel flow related. I also know from diassembling my car that there is an altitude compensator device under the dash, driver's footwell, near the DME; it'll have a ND/Nippondenso label on it. You could check whether this is plugged in, or not.

Best of luck to you, John

[jjeffries]

Also, posting the emissions test results might well be useful.

Checking the cam timing is something I'd do, too.

[rbuswell]

Originally Posted by jjeffries

We both have 82 SC coupes and 87 924S's. Different colors, though.

You'll need someone with more depth than me to give you the best advice, but I'd offer that the FP Relay is a car runs/car doesn't run component. Did the P.O. also operate the car at 6200' elevation? Do you have a fuel pressure gauge you could have hooked up while driving to see if pressure falls at the same time as the stumble you're encountering? That might be a good way to ascertain if this is fuel flow related. I also know from disassembling my car that there is an altitude compensator device under the dash, driver's foot well, near the DME; it'll have a ND/Nippondenso label on it. You could check whether this is plugged in, or not.

Best of luck to you, John

Good taste in cars and model years!

I have a fuel pressure gauge but I don't think it can be stretched into the cockpit of the car. I didn't know there was such a thing.

The car was bought from an owner in Virginia. He never mentioned that it was run at high elevation although I know it crossed the country a few times. He's now dead. Sad story.


I will look for the altitude compensation device.

Originally Posted by jjeffries

Also, posting the emissions test results might well be useful.

Checking the cam timing is something I'd do, too.

The emissions report wasn't where I thought it would be but I will look for it.

If by cam timing you mean that the cog was on its mark when the belt was replaced I'm pretty sure it was.

I just drove the car again and something that may or may not be a clue is that if I'm driving steady in third gear and push the go peddle hard it shows good power and acceleration. It does feel like it's missing at other times while accelerating or cruising.

[jjeffries]

I’ve seen pro mechanics run their fuel pressure gauge up out of the back of the hood, so they can see it thru the windshield; duct tape may have been involved ...

I guess another thing to look for is vacuum hoses in the engine bay ... to be sure they’re all connected and not creating a vacuum leak when you get on it.

We need some of the more experienced guys to chime in.

John

[djnolan]

To check fuel pressure: Fuel Pressure - Checking

Not sure if this would rule out a partially clogged filter and fuel delivery problem but a good place to start.

Vacuum line routing here: Fuel and Vacuum Line Diagram (1985.5 and Newer N/A 944s)

If you have a fuel pressure gauge that doubles as a vacuum gauge I think it should read around 17 Hg at idle.

[rbuswell]

That's a thought although I honestly don't know what vacuum hoses there are on that engine, only that I've never touched them. If they are leaking that could cause this problem.

[rbuswell]

Originally Posted by djnolan

To check fuel pressure: Fuel Pressure - Checking

Not sure if this would rule out a partially clogged filter and fuel delivery problem but a good place to start.

Vacuum line routing here: Fuel and Vacuum Line Diagram (1985.5 and Newer N/A 944s)

If you have a fuel pressure gauge that doubles as a vacuum gauge I think it should read around 17 Hg at idle.

The filter is a Mahle and has nearly zero hours so unless the tank was totally full of sludge, which is a possibility, that doesn't seem likely. I drained the tank when I first bought the car since it had sat for a few years. It looked clear and I even ran the fuel in our lawn mower. Time to crack open the bible we all know as Clark's Garage. It's also befuddling why it happens in a fairly narrow range of driving.

Given the age of the car and having no idea how old nearly all the consumable parts are, I'm tempted to start throwing parts at it. I'd like the car to be my daily driver and road trip car (i.e. reliable). I'd replace a lot of that stuff anyway.

[djnolan]

Originally Posted by rbuswell

..I'm tempted to start throwing parts at it...

Books have been written on this. I would learn to troubleshoot, and buy the tools such as a multi meter, fuel/vacuum tester. Many of the parts will work fine for decades. It's the seals and rubber items that will get you more times than not. Always replace any rubber items while you are in there. The electrical and vacuum stuff is fairly reliable in my opinion.

I only got stopped cold a few times in 10 years, once for a radiator hose and another from a leaking fuel injector body and another from a leaking fuel line. You do know about the timing belt?

[rbuswell]

Originally Posted by djnolan

Books have been written on this. I would learn to troubleshoot, and buy the tools such as a multi meter, fuel/vacuum tester. Many of the parts will work fine for decades. It's the seals and rubber items that will get you more times than not. Always replace any rubber items while you are in there. The electrical and vacuum stuff is fairly reliable in my opinion.

I only got stopped cold a few times in 10 years, once for a radiator hose and another from a leaking fuel injector body and another from a leaking fuel line. You do know about the timing belt?

I'm glad you've had such good luck over the years. I've kept cars running well into the low 300k miles with even fewer problems. This car is a rust-free 924S that is pretty straight but suffered under the ownership of an idiot that abused the poor thing. It was a shame to let it go to the boneyard and I wanted to bring it back to life. Almost nothing worked and it was the perfect example of everything that goes wrong on the 924-944 models.

Yes, timing belts listed in the parts already replaced in my initial post as was the fuel filter. I've replaced a lot of other parts that I didn't think were germane to this problem. Have all the tools. Also have complete manuals for the car. Understand hoses and rubber. Worked on a lot of cars for decades. I don't consider a car old until it hits the quarter-million mark.

My initial post was focused on the narrow range that the engine acts up and why it's sporadic. Curious what your troubleshooting guide says about loss of power only in high second gear and third gear above 2000 RPM but good power otherwise. I haven't found anything about that yet which was why I posted the request for help.

It seems like it should do what it's doing up and down the rev range if it's a failure of the common parts or low fuel pressure. Thought I might get more precise guidance. Perhaps my description wasn't precise enough to get that. "Consumables" include rubber and hoses in my vernacular. But it could also include the fuel pump and regulator which can fail in a 33 year old car and are hard to test for sporadic behavior. I've had them fail in much younger cars with well-made parts.

I hope that helps get us closer to a solution.

[jjeffries]

I'd work to capture the best snap-shot possible of the problem as-it-happens. That was why I suggested the fuel pressure guage. If we were dealing with my 82 SC (totally different), i'd look at the AEM gauge I installed when first tuning the car to see what the wideband 02 sensor was saying at that moment (following my instinct that your car is going momentarily lean). Fuel pressure gauge: engine inputs; reading the exhaust: engine outputs. If this was a car with a distributor, I'd look at the advance mechanism, but Motronic controls that.

Re what Mr. Nolan said, I perceived it as "Let's do our best to understand this, precisely so we don't just "throw parts at it", the correct sentiment, IMO. This is a great forum with some very kind and generous folks sharing wisdom and suggestions.

John

[rbuswell]

"Books have been written on this. I would learn to troubleshoot, and buy the tools such as a multi meter, fuel/vacuum tester." doesn't read the way you see it. It says you don't know anything about cars and make the investment before you come back here. What I see is a guy who didn't read the thread. He told me to do two things I said were already done in the first post. That doesn't sound like a guy who wants to understand. I gotta tell you, I'm really struggling with apologizing for taking offense at rude comments.

One of my best friends in the whole world was Grady Clay. If you never heard of him, check it out on Pelican. He is considered a legend. Grady would never say something like that to anyone ever! He was the model of how to conduct oneself on Pelican.

[rbuswell]

All hoses are plugged into the correct Ys and sensors from what I can tell. I did not take each hose off and test it for leaks. That seems to be counter-indicated since the symptom only happens in a narrow RPM and gear range. In my experience, cars don't run well across the board with vacuum leaks or when hoses are plugged into the wrong port. If any Pelican has experience to the contrary, I'm open to rebuilding the vacuum hose array if needed.

I thought my fuel pressure tester had a higher PSI range than it does so I have ordered another one. Expect it by tomorrow. I may still need to build a fuel rail adapter per the tutorial in Clark's Garage. Didn't want to go down that path until I know my new tester doesn't include the right adapter first.

Found a sentence in Clark's write up on fuel regulators that gave me hope that the regulator may be the culprit. It didn't describe my car's symptoms exactly but sounded close.

[thomasryan]

I would look at the wiper tracks on the afm. you can file the mounting holes and slide the board over a little.

at your elevation, your fuel trim should be in the negative teens. try to run it to the low fuel light and tank up with the next higher grade.

does it flat spot when the engine is cold? if not I would look at the emission stuff and its operation.

[rbuswell]

Originally Posted by thomasryan

I would look at the wiper tracks on the afm. you can file the mounting holes and slide the board over a little.

at your elevation, your fuel trim should be in the negative teens. try to run it to the low fuel light and tank up with the next higher grade.

does it flat spot when the engine is cold? if not I would look at the emission stuff and its operation.

1. I'm frankly kind of chicken to drive it for long distances until I solve this. I'd say my longest drive was about 40 miles round trip. The flat spot is immediate upon starting and driving. It doesn't seem to go away after being fully warmed up on my few longer drives either.
2. I don't own a tool for reading trim. Any recommendations?
3. Can you elaborate what wiper tracks are? Sorry for my ignorance.
4. In Colorado there is only one grade higher fuel which is 89 octane. All Colorado fuel is also pretty high in ethanol. Nearly full tank but I can drain it and put it in one of two other cars that use 87. There are also octane boosters I can add.

[rbuswell]

Originally Posted by rbuswell

1. I'm frankly kind of chicken to drive it for long distances until I solve this. I'd say my longest drive was about 40 miles round trip. The flat spot is immediate upon starting and driving. It doesn't seem to go away after being fully warmed up on my few longer drives either.
2. I don't own a tool for reading trim. Any recommendations?
3. Can you elaborate what wiper tracks are? Sorry for my ignorance.
4. In Colorado there is only one grade higher fuel which is 89 octane. All Colorado fuel is also pretty high in ethanol. Nearly full tank but I can drain it and put it in one of two other cars that use 87. There are also octane boosters I can add.

Found the AFM procedure in Clark's Garage. Terminology makes sense now. He didn't use the word trim in his description though.

[thomasryan]

AFM is what meters/measures the air the motor is consuming and its temperature. the first rendition was on the VW 411. there is an arm attached to the 'barn door' or flapper that makes contact on a carbon pile style surface. when the parts wear, the computer cant tell how much air the motor is ingesting.

at elevation, the air is less dense so less oxygen. therefore you need less fuel which results in less power at a given rpm. higher octanes have slower flame fronts but you might try a different fuel grade and type. if I cant run 100% 93 from co-op, shell premium is my 2nd choice.

imho..ethanol is a fallacy much like fluoridated water.


don't worry about trim.

[rbuswell]

Originally Posted by thomasryan

AFM is what meters/measures the air the motor is consuming and its temperature. the first rendition was on the VW 411. there is an arm attached to the 'barn door' or flapper that makes contact on a carbon pile style surface. when the parts wear, the computer cant tell how much air the motor is ingesting.

at elevation, the air is less dense so less oxygen. therefore you need less fuel which results in less power at a given rpm. higher octanes have slower flame fronts but you might try a different fuel grade and type. if I cant run 100% 93 from co-op, shell premium is my 2nd choice.

imho..ethanol is a fallacy much like fluoridated water.


don't worry about trim.

Ethanol is not a fallacy, it's a fact of life for those of us who live here. The robber barons get about $.30 a gallon more for each step up. We put up with the greenies as a cross we bear for living in paradise. They are convinced ethanol reduces our brown cloud. There are a few ethanol free stations but they don't carry it all the time. I live about 20 miles from Denver. The car is going to be a daily driver so pump gas close by is a necessity since it's a long drive to get anything better.

[rbuswell]

I had a long list of other things to do to the car and was finally able to drive it for a while today. How it went is a little lower in the post.

A couple notes:

• Bought a pretty good fuel pressure gauge and made my own rail cap per Clark's instructions. The cap leaked a little which I assume is normal since there's no ball bearing making a seal. Next time I use it, I will try Teflon tape. The fuel pressure read perfectly and rose in a linear fashion with increasing RPM just like I hoped.
• Cleaned and adjusted the AFM to make the mixture a little richer (about four teeth).
• Plugs were perfect light tan with no indication of combustion issues.
• I did not do anything about fuel yet since George mentioned that he's never had a fuel issue in the 87 octane range.

Now for the test drive: It started and idled well like before. All gauges indicated perfect. It had pretty good power pulling away but still had a dead spot/slight miss like before ... in second and third gear. Figuring that the other indications were positive, I drove it longer and took it through more driving conditions. The more I drove it the less the dead spot was apparent. The car eventually had good power through all five gears and various rev ranges. Most of the driving was still at around 6,000 feet elevation. I drove for around 1/2 hour and about 30 miles. I had replaced all the suspension bushings, etc. and put on new tires so I need to get an alignment. I did a string alignment and it drove nicely. I didn't want to damage my tires so kept it short.

I took it close to redline a few times and drove to highway speeds (about 80 MPH) and it would accelerate to about 90 MPH pretty easily. I'm careful about tickets since I've gotten a couple lately. There is still a much less pronounced miss but the power in all gears is MUCH better. My plan is to keep driving it to see if the flat spot goes away altogether. I'll also move up an octane range on the next fill just to eliminate that as an issue.

One question though: There is a whining noise that matches RPM. It's not real loud. It has all new belts including the timing and balance shaft belts. Could this be just new belts seating in or something more worrisome? Or something unrelated to belts?

Thanks! I'm jazzed about my new car ... finally.

[rbuswell]

I have made two changes that appear to have taken me much closer to "good to go".

• I switched to premium unleaded gasoline per one of the suggestions above.
• I tested the original Bosch ignition coil and found it wanting as to the factory ohm readings. I replaced it with a new MSD Blaster 2 coil which tested out perfectly as you'd expect with a brand new part.

Those two changes made a world of difference. The car is much more enjoyable and seldom shows the flat spot acceleration I asked about originally.

I have an analog MSD 6AL ignition box I plan to install too. Other Pelicans have made that upgrade with good results. None of the threads I've read tell me how to do it other than to say it's easy. I can dig deeper to find the procedure or get help from MSD. My biggest question is what lead I use to get the trigger signal from the engine. If anyone already knows that answer, it would keep me from starting a new thread.

Thanks again for all your suggestions. They all helped me find the final solution.