View Full Version : Air/fuel problems

12-27-2016, 02:10 PM
Recently my car has developed a rich problem while cruising. It hits a 10:1 air fuel while cruising down interstate or in town. It's intermittent thus hard to diagnose, although it is doing it more often. I've tried 3 different maf s so far and my O2 sensors are new. This goes along with another problem I have on cold starts where it leans out to 18:1 and sometimes dies. Both problems are worse with the cold and the car doesn't have any codes. I've check multiple times for vacuum leaks also.I'm leaning towards the computer, but would like to hear any opinions on possible problems.

sam jones
12-27-2016, 09:30 PM
Good evening

I would do the following:

- Fuel pressure test. Are the pressures within limits? Inspect the fuel pressure regulator and vacuum line. Can you smell fuel? Is the line cracked?

- Start the car and with a scan tool check live data of the O2 sensor "switching". Look for one reacting slowly or intermittent at idle but reading good when the throttle is advance from idle to @2500 RPM. Even though you changed the sensors an exhaust leak at the manifold, exhaust flange and or exhaust pipe to catalytic converter could be just under the threshold of causing a lean condition fault and running rich.

- Do you have a low coolant condition gradually re-filling the recovery tank weekly. The cold start, rough idle, engine shut off are all symptoms. On the next start check the recovery tank for bubbles "trails". This would be an indication that the cylinder head/head gasket is at fault. A block tester would confirm the analysis.

Good Luck.

12-27-2016, 09:40 PM
Thanks for the info, checked the coolant already, I have to add every few months at most and only a little bit. It doesn't do this every cold start either which makes it frustrating. I'll try to re-check the O2's. This is not the time of year I like to do my mechanic work ... my mileage is looking like it will be 12 mpg this tank down from 20 with mixed driving...

12-27-2016, 09:50 PM
I wanted to add, I reset my computer today and the cold start went perfect, with only a few seconds of roughish idle then ran flawlessly. At least until interstate where it ran normally for 10 miles or so then dipped to 9.6-10:1 air fuel, then it went between running normal or super rich on and off. Normal running lasted about 1-2 miles at a time.

12-28-2016, 09:13 AM
Sounds like 02's to me put the old ones back in if they were ok. We never have much luck with 02's from the big chain part stores get them from motorcraft.

12-29-2016, 08:02 PM
Tossed on another set of Bosch O2's and it runs at 13:1 a/f on cold starts until operating temperature and 14-15 down the highway again so far. Not sure what cold start a/f should be since the lowest I've had since my wideband was installed was 14:1. I think I will be investing in Motorcraft O2's before long since those Bosch lasted a whopping 10k miles....

12-29-2016, 08:22 PM
They do run a little rich on a cold start that is normal.

Watch eBay I usually find good deals there on new motorcraft parts

01-08-2017, 07:34 PM
So after driving 200 miles, I get 14.2 mpg. I still see the air/fuel drop in the 9:1 range randomly at cruise. The trips start fine at 14-15:1 a/f then after a while it will drop slowly down in the 12's then 11's then jump back to 14-15 for a while. The only way I've corrected it if it stays rich is to get into boost then let off and the a/f returns to normal. Under boost it reads the tune from the SCT chip and runs perfectly, which is why I'm leaning towards the computer.I'm going to wait for the computer to finish it's learning cycle before I swap the computer out. I'm also looking at new O2 sensors just in case the Bosch were a bad run...With the old motor and stock computer/tune, it would sometimes have these rough starts that would sometimes cause it to die like I'm experiencing now. I never did get that to stop which is another reason the computer might be at fault.

01-08-2017, 08:04 PM
> I'm also looking at new O2 sensors just in case the Bosch were a bad run

Starting to wonder if modern Bosch O2s aren't the best for these cars...perhaps NTK/Motorcraft would instead make a difference, since I have roughly similar symptoms and used Bosch last time.

01-08-2017, 08:40 PM
Still sounds like 02's when you get on it the car does not read the 02 sensors it only uses them at idle and part throttle.

01-08-2017, 11:46 PM
I'm going to toss another set of O2's at it before anything. These random problems piss me off since I'm basically stabbing in the dark and can't even trust new parts. It almost makes me want to put a stand alone computer in and just have somebody tune that lol....

sam jones
01-09-2017, 01:18 AM
Good evening

Might be two issues.

- Does you car have a EGR? The tube could be blocked with debris and or the bellow could be cracked causing a leak.

- Does the car have a custom exhaust? If so the problem might be the modification of O 2 sensor to fit the new location of the bung. I have a 1989 LX I converted from MAP to MAF function.

I had an exhaust shop modify 96 thunderbird exhaust manifold and re-due exhaust from flange to muffler. O 2 bung was move further back than the stock position which required the O 2 sensor cable to be extended.

I had never ending issues with intermittent check lights, scan lean codes. I finally replaced the sensors with new longer stock Mark VIII units. No more issues with check lights, poor mileage or running rich.

Lessen learned. Don't modify O 2 sensors (splice and solder) to fit the car.

As an observation I noticed BOSH sensor "bullet shield" does not "self clean" if you find signs of contamination. The DENSO "Swiss cheese shield" does a better job.

Good Luck.

01-09-2017, 09:09 PM
My 1990 auto luckily doesn't have an egr and although it's a custom exhaust we reused the portion of the stock exhaust with the O2 bung in it, the other side is in the exhaust manifold itself.

sam jones
01-10-2017, 05:56 AM
This goes along with another problem I have on cold starts where it leans out to 18:1 and sometimes dies. Both problems are worse with the cold and the car doesn't have any codes. I've check multiple times for vacuum leaks also.I'm leaning towards the computer, but would like to hear any opinions on possible problems.

Good morning

Have you checked:

- The Air Charge Temperature Sensor for possible re-sealing?
- The inlet air supercharger manifold for re-sealing?
- Driver side cylinder head valve cover PCV hard plastic hose for damage?
- The fresh air side of the PCV hose is hose connection still attached to the intake duct at @ 6 o'clock position?
- The "Y" metal PCV metal manifold for cracks (back of the supercharger)? Any soft emission hoses?
- Vacuum block tree to intake manifold inlet elbow hose for cracks/damage?
- Charcoal canister solenoid emission hose for cracks/damage?

01-10-2017, 09:33 AM
Smoke test it if you can..... That would reveal any kind of leak you may have...

01-10-2017, 10:07 PM
Unfortunately last time I tried to get it smoke tested I found that no local mechanic had a working smoke machine.... I've already checked the usual suspects for vacuum leaks thou and everything is good. I have the MP plenum so most of that vacuum system is new rubber hoses.

sam jones
01-10-2017, 11:00 PM
Good evening

Make your own DIY evap smoke tester. It will pay for itself the first time you discover that previously unknown leak. Here an example:


Good Luck.

01-20-2017, 11:14 PM
I installed the computer I had sitting around, so we'll see. While I was swapping the cover over from the old one I noticed that the new one appeared to only have needed one capacitor fixed by cardone. I then looked at the same capacitor on my computer and it had one silver soldered leg and one that looked burned. There was also a yellowish colored liquid on the component below that capacitor. Hoping this fixes it or at least some of the problems.

02-22-2017, 11:04 PM
So all was well for 150 miles on the new computer learning session, then the same symptoms started showing up during regular cruising(random super rich conditions sometimes reaching 9.5:1 a/f) I replaced the O2's with what is now my 4th set with no change. I had a video taken of the offending behavior in town driving in a 40 mph area. If you watch the video you will hear my annoying exhaust get loud for a second. This was a quick mash of the pedal. This is the only way I can seem to snap the car out of the rich condition, and only lasts temporarily for random periods of time.... It will sometimes run normal for a full trip, others only occasional dips in a/f. Here's the video https://youtu.be/qd1_Siy4rZo