Okay. Here is the car, its a '97 Cobra with o/r x-pipe, MIL eliminators, flowmasters, 4.10's. Other than that its pretty much stock. I bought it a few months ago from a friend. Its got 95,000 miles on it and wasn't taken very good care of, but the price was right. The CEL has been on from the day he bought it, 3 years ago. So he just drove it around like that for 3 years and the car feels very slow for a Cobra. I haven't had much time to do anything to it, other than replacing the AC compressor. But now I am in the UAE and I left the Cobra with a friend so he could fix and drive it. He's a competent mechanic and owns an '04 & '05 GT so he is somewhat familiar with mustangs.

So here is where it started a week ago. He has a code reader so he downloaded the code and it was a P0171. So first we changed the front O2 sensors (because they have 95k on them). The P0171 came back. Also because the car is old we Seafoamed the intake. After Seafoaming and clearing the codes, the P0171 came back and now a P0174 came up. Common sense would tell you that if you got both codes, there must be a vacuum leak. So after a short check, it turns out the Seafoam worked too good and cleaned out all the gunk that had been sealing the intake. So we changed all the intake manifold gaskets and cleaned the IMRCs and checked the exhaust manifolds for leaks and tried again. This time the P0171 and P0174 came back and then a P1443. Good thing is all the lost power is back, now the car pulls smoothly all the way to redline. He checked the Charcoal Canister and all the vacuum lines, the EGR system , gas cap and PCV valves. Found a cracked up line and replaced it and the 1443 went away and hasn't come back but the P0171 & P0174 where still there. So we checked for intake leaks again and found small leaks at the ends of the IMRC's. So we pull the intake back apart and replaced the seals on the IMRC shaft and put it all back together.
After rebuilding the IMRC's and finding the IMRC cable had broken so he fixed that and now only the P0171 came back and its only tripping at idle. We're going to wait a couple days and put some miles on it and make sure only the P0171 is there. I'm going to have him check/clean the MAF and if that don't do anything, we are going to pull the fuel injectors and clean them or possible just replace them.

So that's it. After all this, we started with just a P0171 and have worked our way back to just a P0171. But after all this the Cobra acts like a completely different car, pulling hard from idle to redline. Unlike before. It still has an issue with the idle, after the engine is warmed up, and you drive it around. When you come to a stop the RPM comes down to about 1000-1100rpm and after about 5 seconds drops to 600-800rpm.

So has anyone had something like this before? Any help, advise or opinions would be greatly appreciated. Thanks.

 

someone over here in germany is having the same problem with his 2001 cobra. you guys are hitting all of the stuff that OBII is telling you to on their troubleshooting list.

your RPMs hanging could be caused by the IAC not functioning properly.

clean the maf inspect the intake inlet hose for cracks.

how did your spark plugs look? does it appear your car is running lean or rich?

 

Well, we cleaned the MAF, checked the IAC, cleaned and leak checked the Injectors, and rechecked the manifolds. There is still a tiny leak at the IMRC bearing, but its tiny. We're going to try and fix that some how. Not trying to delete the IMRC's just yet. Unless someone knows a place in Phoenix that will tune it for a reasonable price afterwards. Right now our main concern is getting the car smog-legal again so we can get it registered. We checked the plugs and all are fine, ohm'd all the sensors and they are all good. A new fuel filter is going in tomorrow and we are still looking for a fuel pressure gauge so we can check the psi. We had two gauges and one reads at the lower end of what acceptable and the other gauge reads 8psi lower than that. So we're going to change the filter and see if there is a change in the psi. But that tiny leak at the IMRC bearing is still going to need to be fixed. So if we cant come up with something to temp fix the leak long enough to get it smogged, then i guess the Delete is the only option left, since there is no guarantee that swapping the IMRCs with other used IMRCs wont still have leaks.

One good thing out of all this work is that the car is outstanding again. Able to roast the 315's all the way into 3rd gear, when before it would barely brake them lose in 1st gear.

 

This is a too lean Bank 1 code. I would try this first, it wont cost any money just a little time. Swap all the fuel injectors from the right side to the left and vise-versa. If you come up with a P0174 then you will know that there is a problem with the injectors, maybe one or several are not spraying the way they should.

 

Well, the P0174 is back now. We put a new FPR, MAF and fuel filter. The pressure is 6psi low with the Vac' connected and 8 psi low with it disconnected. Was talking with an ASE guy I know, and he thinks that the IMRC bearings leaking may not be enough to trip the codes this consistently. The codes only pop at idle still, only twice out of the 30ish freeze frame data's that I looked at showed it tripping at about 2500 RPM. So next up I am going to change the fuel pump, which is also a good time for a 255lph upgrade. We found tons of places online to get sealed bearings, so if the pump dont fix the CEL's, we’ll just have to take the intake apart again to mic the inside and outside diameters of the bearings. Even if the pump fixes the codes I would still do the bearings or delete down the road shortly.

Here is the parts list so far...
Forward O2 sensors
Intake Manifold gasket set
Intake manifold & IMRC cleaned (looks like new on the inside)
IMRC cable replaced (driver side)
IMRC o-rings replaced (both sides)
New MAF
New Canister purge valve
Cleaned IAC (was gummed up and making humming noise)
Cleaned Injectors
FPR
Filter

Next up are....
Fuel pump
IMRC bearings

I was trying not to just throw money at this problem but that's just the way its ending up... Been about 3 weeks since we started trying to fix the issue.

 

Okay here goes, all at idle.

Connected Vac on FPR: 30-45 PSI fuel press required, mine was 24 PSI
Disconnected Vac on FPR: 40-50 PSI fuel press required, mine was 32 PSI

Blocked off return to the tank: supposed to be super high compared to the other numbers, it was but it wouldn’t hold once the key was turned off so it is either bleeding back through the pump into the tank or leaking out the injectors, and since we don’t have a rich code or problems starting cause of flooding, we’ll go with the pump. So i ordered a BBK 255lph and it should be here in a couple days.

 

Did you swap injectors side to side? and code P0174 came up or did 174 & 171 come up together?

 

Both the codes are back, without swapping the injectors. I had the injectors out last week and professionally checked and cleaned, so I'm pretty sure they are not the issue.
At this point its either the IMRC bearings or the Fuel Pump. Both of which I will be replacing this coming weekend.

 

Well, the Fuel Pump is in and the pressures are all good now, but the 171 and 174 are still there. Only thing left it can be is them IMRC bearings.

 

Well, no good news yet guys. He changed out all the seals and IMRC bearings and they are working out good and are not leaking at all now. But the codes are still there and now the car wont idle properly. Installed a new IAC since the old one was gummed up and that didnt help, installed a new TPS and that didnt help either.

We're running out of things to change/clean/fix/inspect. At this point we are going to replace all the old vacuum lines with some new rubber ones. We did a pressure check on the system and it wont hold pressure, so its either got cracked/broken lines or bad components that the lines go to. But since the rubber hose is cheap, we will start there.

Other than that, only things left to change are EGR (valve, vacuum solenoid, and pressure monitor), air/smog pump, fuel injectors.

He is going to replace all the o-rings on the fuel injectors this time, since he has to pull in intake again to replace all the vacuum hoses and to make sure something didnt happen or get over looked on the last install, which might cause the idle problems.

 

I had a similar problem on my F150. It was a combo of bad vacuum lines and bad egr. Did you hook a gauge up to see vacuum at idle? This would tell you right off the bat if you had vacuum leaks.

 

2 ?'s, you said you replaced the O2's but whos' did you use? aftermarket or oem?
also on the top of the throttlbdy there is a black rubber cover, which covers a brass screw which you can turn in and out to help control IAC idle, make sure that screw is still there and that its seated, maybe even try adjusting it?

 

Ya the vacuum with engine running was okay, but with the engine off the system would not hold pressure, except the part that goes into the passenger compartment. Which is why we replaced all the vacuum lines with new rubber ones and got rid of all the plastic ones. We haven't replaced the EGR yet because it checks good, but we'll see. May have to look at it again to be sure.

We used oem style Bosch. I didnt know about that screw on the TB, we'll give that a look. The main problem there is that when you let off the throttle the RPM's come down and stop at about 1500 and sit for a few seconds and then drop to idle.

 

well I've seen problems outta' the Bosch O2's, so maybe you outta' look into getting some oem Ford ones and who's thrttlbdy is it, stock Ford or aftermarket?

 

Hum. I've only used Bosch o2's for years and have never gotten a bad one before. And the TB is stock.

We're going to change the Differential Pressure Feedback EGR (DPFE), we had checked the EGR valve and sensor already and they checked good, but didnt know about this DPFE and it checked bad. Apparently if its bad it can send false info and operate the EGR when it should not be or should be and cause a lean condition.

One thing to keep in mind about all of this is that the lean condition is only at start up... Not at idle or anywhere throughout the RPM range, just on start up. Its not running lean, so i dont think its a major vacuum leak. Its got to be a component that isn't doing its job. So i dont know what else to look at, we're at ropes end on parts to check and change.

 

It might be a long shot but it could be the O2 not heating up fast enough and giving a bad signal.