951 indicating very lean, won't run

[mytrplseven]

Got my 951 back together. It was a barn find and it ran after it was delivered to me in Florida. After putting some fresh premium in it I put it up on jacks and tried to start it again and it wouldn't run without spraying starter fluid into the intake. It barely ran and wouldn't accelerate. So I got started with everything I could think of to make sure that when it finally got fixed, everything would be right. I replaced the entire ignition system, rebuilt the turbo, installed AFR and turbo boost gauges, new silicone vacuum lines, replaced a couple of sensors under the intake manifold that were being bypassed, rebuilt injectors, new fuel pressure regulator and damper, new fuel lines, filter and today a new fuel pump because I was feeling it was a fuel delivery issue. I also cleaned the throttle body and checked the TPS and replaced the speed sensor because the connector was bad. Firing order is correct and there's spark getting to the new plugs. The AFR says the engine is running very lean and when it struggles to run, it barks (pre-ignition).

I can't seem to get to the bottom of what's going on here. I'm chasing my tail and don't want to hurt the motor. The timing belt is right on the marks and I even got rid of an intercooler that had a leak in it. The last time I had a 944 N/A that ran like this, the AFM was disconnected but this isn't the case here. It has an OEM chip and I was meticulous with my engine work. It's just running lousy (technical term) like it was right after I refueled the car. I opened the fuel line from the tank and the fuel flowed freely so I don't think I have an issue with that.

I hope someone out there has the answer to this problem. I'm stumped.

[kdjones2000]

Rather than doing the shotgun approach, start with a proper strategy for debugging your issues.

1) Check your compression
2) Check your spark & timing
3) Check your fuel pressure (yes, get a proper gauge, not what you described above)

Go back to Clark's garage and follow their procedures, get the DME Test Plan and go through that.

[mytrplseven]

I have a pressure gauge on the fuel rail, Spark is good and timing is spot on. This thing started up with no warning. All I did was fuel the car and put it on the stands. I was planning on re-doing and replacing all the stuff that I listed anyhow so I'd have a reliable ride. Unfortunately, whatever is causing this wasn't part of that. AFM wiper on the track is very erratic and all over the place. Reforming the wipers didn't solve the issue so I have another one coming.

[TrailRider]

Does the car just crank or does it try to fire / bog then die?

Is the tach bouncing as the car cranks?

[mytrplseven]

It tries to run and fire but then dies. The tach bounces. I ran tests on the AFM and the resistance rise on the wiper should have been consistent but it wasn't so I'm getting a replacement one. I tried to fix the tracks but it didn't work. I guess I'll pull the DME connector as today, as well and make sure I'm getting the proper signals at the connector.

[kdjones2000]

Have you pulled your fuel rail and checked volumes out of your injectors?

It sounds like you have some clogged injectors, based upon what you have described.

Again, check for the basics first. If those are all good, then worry about the DME Test Plan.

[mytrplseven]

HMMM. Interesting question. Although they're new and protected by new filter, I wonder if anything got in there when the fuel lines were replaced. What's the best way to check them without spraying fuel all over the garage and my car?

[bpu699]

Check your fuel pump relay...Could be intermittent...

[mytrplseven]

Can you believe it? I decided to go back to basics (on suggestion of KDJones) . I did a Noid check and the pulses were getting to the injectors. What I thought I saw as tach bounce was, in fact, the engine trying to run. So I decided to pull the connectors for the Ref/Speed sensors and check the resistance. The Speed sensor had 0 ohms when it should have been 600 to 1600. The ref, on the other hand, checked out perfect. Well, this is going to be fun....at least I have something concrete to work on. I should have replaced the Speed sensor when I replaced the Ref sensor that had a bad connector. I figured since the car was running, it was ok......my bad.

[TrailRider]

Great, glad you possibly found the root cause. Let us know how you get on.

[DannoXYZ]

Measuring things and getting data (numbers) is the key to fast and efficient troubleshooting. Most of the parts you replaced were probably perfectly OK.

Here's some more measurements to take:

- fuel pressure at idle with vacuum hose disconnected
- fuel pressure at idle with hose connected
- AFM voltage at idle
- intake-manifold vacuum at idle with warmed-up engine (looking ahead to when the car actually idles).

[mytrplseven]

Originally Posted by DannoXYZ

Measuring things and getting data (numbers) is the key to fast and efficient troubleshooting. Most of the parts you replaced were probably perfectly OK.

Here's some more measurements to take:

- fuel pressure at idle with vacuum hose disconnected
- fuel pressure at idle with hose connected
- AFM voltage at idle
- intake-manifold vacuum at idle with warmed-up engine (looking ahead to when the car actually idles).

Found bad speed ref sensor. Also still suspect of AFM due to erratic rise in resistance of wiper over track. New fuel pump, filter, lines, FPD and FPR and a permanent gauge on fuel rail. Pressures are now spot on. Engine won't idle so can't do the other tests. We'll see how she runs after the new speed sensor comes in.

[mytrplseven]

Originally Posted by mytrplseven

Found bad speed ref sensor. Also still suspect of AFM due to erratic rise in resistance of wiper over track. New fuel pump, filter, lines, FPD and FPR and a permanent gauge on fuel rail. Pressures are now spot on. Engine won't idle so can't do the other tests. We'll see how she runs after the new speed sensor comes in.

I'm pretty sure the speed sensor was BAD!! Looks like I'll have to recheck the clearance, too. Wow!

[kdjones2000]

Things like that have caused hot start issues for people I know.

Put in a new one...

[TrailRider]

Ouch... yup, time to replace.

[mweinst]

In regards to the AFM, it was my understanding that the voltage should change smoothly when the flapper is opened, but that resistance is usually inconsistent, making it a poor indicator of AFM health. Is this incorrect?

[mytrplseven]

Originally Posted by mweinst

In regards to the AFM, it was my understanding that the voltage should change smoothly when the flapper is opened, but that resistance is usually inconsistent, making it a poor indicator of AFM health. Is this incorrect?

I'm not sure about that. I'm waiting for a supplier to do a resistance test on a known good turbo AFM to see if the rise in resistance is smooth and constant as the barn door is slowly opened. The AFM from an N/A that I have was a smooth, constant rise in resistance and I can't imagine the turbo is any different.

[mweinst]

Please let us know what you find out. I've been chasing a lean surging on part throttle acceleration, and my AFM resistance jumps all over the place. I read that it didn't matter, but if it does, that may be my issue.

[mytrplseven]

Electronically, to me it makes perfect sense. Voltage values are a direct product of resistance applied to them. It the resistance increases on a constant, linear curve, so too should be the voltage change. I talked to my supplier and he said the one he checked for resistance consistency was just as I expected, so I bought it.

Stay tuned.

[mweinst]

I get what you're saying, but the voltage on my AFM is as smooth as glass. It's just the resistance that's not right. Anyway, maybe this will fix both of our problems. Please post back here when you find out if it works for you.