BrianRMV

I have a 1986 951 with about 201k. A few months ago I had a mechanic take a quick look at my turbo. He pointed out that the shaft has some play left to right, and a bit more in and out. He said the more serious problem is the in/out play. I'm seriously thinking about having a rebuilt turbo installed. I generally have "unsteady" acceleration. It's essentially acceleration with cyclical drops in power. I also noticed recently that when I keep the accelerator fixed, on a flat road, I feel a slight surge up and down in speed. It's not much, but noticeable if I'm looking for it. I'm wondering if this could be a result of my turbo failing. Am I off here, or does this seem like a reasonable cause and effect?

Thanks for any input.

Fastbackjack

With 200k, it could be anything. MAF (groove in the board / poor contact), TPS, injectors, turbo, wastegate, etc. In regards to the turbo, is it holding boost while at full throttle and under load?

BrianRMV

The MAF is "new" used. It was installed about 2 months ago. It was purchased and installed by a mechanic, so I don't know anything about the vendor. It's definitely worth looking at. The TPS is literally brand new and OEM. The injectors have under 5k on them and the current issues were not happening until recently. I suspect that the turbo is getting "tired". The wastegate has not been inspected. I took the car out last night and had a serious stutter early on under load, and it had what seemed like a series of mild backfires that lasted until I let off on the throttle. I decided to keep driving it very conservatively and it seemed to run okay. At the end of the evening when I went home I decided to accelerate at near WOT, and it ran well. The problem seems to be sporadic, the severity anyway. I'm not remotely qualified to run most diagnostics on the car so I'll probably drop it off at my mechanic's and hope that the behavior repeats itself. One thing I've wondered about but have not confirmed is if it seems to be more of an issue before it warms up. I have had trouble with the idle air control valve in the past.

Thanks for the suggestions. If anything else comes to mind please let me know.

Fastbackjack

Coolant temp sensor may be an issue. Gonna have to go sensor by sensor to make sure everything is reading correctly to rule everything out. I'd start with checking everything before replacing a turbo.

Jfrahm

It could also be a bad knock sensor or dodgy fuel. timing coming in and out.

BrianRMV

Thanks for the ideas. I have wondered about it being a fuel issue, but I've been refilling at the same station for years. I've also wondered if there is any way I could have some water in the tank but think it's highly unlikely. Even though my turbo may need a rebuild fairly soon I don't think it has anything to do with current running problems. The thing that seems really odd is that when I got it back from the mechanic about 3 months ago I had the problem the next day. I took it to a new mechanic and left it with him. He never experienced the problem when he drove it. When I took it back it ran well. 6 weeks +- later and the problem has popped back up. I'm sure the randomness of it won't help make it easier to diagnose.

Wisconsin Joe

What are you using for gas?

If you are using E10, then the possibility of water in the fuel is practically zero.

The ethanol will mix with the water and it will burn through the motor.

If you insist on using 'non-ethanol' gas, you could run a tank or two through it, or get a bottle of 'Heet' gas line de-icer. They both are alcohol and will take care of any possible water.

BrianRMV

I do use E10, only because non-ethanol is hard to find and expensive here. I agree that the gas is unlikely a cause. My new mechanic thinks something may be wrong in the ignition system, which does not narrow things down much. One thing I need to confirm is whether or not the issue is worse before the engine warms up. Checking the coolant temp sensor seems like a good idea. The knock sensor was replaced a couple of months ago, but I've heard that even new ones fail fairly commonly. If anything comes up I'll post it.

Thanks for the input.

Noahs944

It is possible that the turbo is faulty, unfortunately this is where modern electronic system are much easier to diagnose.

Jfrahm

If it does not do it when the engine is pretty cold it might be timing pullout due to an overzealous knock sensor or too-low fuel octane. Actually it could be a bad knock sensor in either case, I have had them fail where they detect a lot of knock that is not there.

Back when I had a Mustang that'd run OK on midgrade but ping if the octane was a bit too low, I'd get ping on a tank of gas every now and then, even from the same gas station. I had to get a few gallons of premium to sort it out. It can happen that the fuel octane out of the pump is not what you expect.

BrianRMV

I've done a little closer observation on my issue. I have noticed that at wot it seems to accelerate smoothly, but at less than wot it does not. If I accelerate hard in first I frequently get a small backfire when shifting to second. My first thought is that the air flow meter might be bad. I wondered if the fqs could be in the wrong position, but I have an A-Tune, so I do not think it affects timing anymore. I'll make sure it's in the default setting though. I have had problems with the idle air control valve in the past. It was primarily noticeable when the car was cold. It would hunt badly and occasionally die until it warmed up. Since I got it back from my previous mechanic it seems to be working well, but I wonder if it could possibly contribute to my current running issues.

Has anyone had a bad experience with the A-Tune? I really doubt it's part of my problem. I don't think I've ever read anything bad about it, but would appreciate feedback (good or bad) about it.

Thanks

BrianRMV

I've done a little testing on the slight stutter issue and have concluded that it starts after the engine has warmed up. It warms up very quickly, so I did not pick up on it previously. For the first 2 minutes +- it runs smoothly with steady throttle. After that it has a slight surge up and down. It's fairly minor, but definitely noticeable. The obvious culprit (I'm guessing anyway) is the DME temp sensor. Does that sound right, or could it be something else?

Thanks

BrianRMV

Update -

I have no idea why, but the stutter went away a little over a month ago. No more stutter and minimal if any backfire (the backfire was never major). I expect it to come back some day, but for now the car is running really well. Gas mileage is up a bit too. Still not great but close to 19 mpg on the last tank.

bonus12

The intermittency of the problem is confusing. Maybe you can consider installing an air:fuel ratio gauge to allow you to monitor that aspect. Also, I am going to make a controversial recommendation, which is to run some Seafoam, Techron, or Gumout through your gas tank. That would help clean the fuel lines and injectors and would be especially beneficial to a car with 200+ K on the clock. It doesn't sound like a turbo issue. It sounds like air:fuel mixture problem to me, with quite possibly the combined effect of a faulty electronic/computer input. I wish I knew more about the DME Temp sensor, but hopefully someone else will provide input there.

Have you checked the connection to the cycling valve? Maybe a connection is halfway loose? It's under the intake manifold in the rearside of the engine bay. You can see it from the top.

Another thing I want to ask is when you last did a tune-up? That couldn't hurt any. Spark plugs, dist cap and rotor, air filter, fuel filter...
I kind of doubt you are dealing with a single issue. Instead, it might be a variety of minor things.

BrianRMV

Thanks for the input. I have considered installing an air/fuel ratio gauge for a while. Any recommendations on brand/model? A friend suggested running Seafoam through the system but I've always been reluctant to add additives to the fuel. Is there any potential damage it could cause?

The cycling valve failed years ago. I was not getting boost, so I purchased a manual boost control through LR. I have always wondered if the cycling valve could still have an influence on running.

Tune-up and fuel filter were done a couple of years ago. The air filter is kind of a long story, but a recent mechanic lost the air box, so he put in a shop built filter system that I'm not at all happy with. I will be putting a new air box in soon, but the price is really prohibitive for new, and used ones are amazingly expensive. Hopefully I can find a reasonably priced one soon. If not I'll add a new one to my list of inexplicably expensive parts.

Thanks again for the suggestions.