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I;ve been working on a “97 thunderbird. It’s been running rough, and smelling like it burns a little bit of oil. It never threw any codes at me till last week. It gave me P1151 HO2S detects lean. I have ruled out EGR problems, fuel pressure, Mass air flow, and didn’t find any leaks with the torch. I decided to do a compression test to see if it was worth throwing parts at to try to solve this very non-discript code.
The compression test came back with all 8 cylinders fairly close, but MUCH lower than what I anticipated. (59-69 psi.) I haven’t been able to come up with the compression ratio of this engine, but those numbers seem lower than what I would expect. Does anyone happen to know what those romeo 4.6L SOHC engines should test at?
Also, when pulling the plugs I noticed a little bit of carbon on all, but a few of them had ash deposits on them (and they’re not very old). This brings up the question of what is the likelyhood that a valve problem is causing the ash, low compression, and burning oil? How about a lean code? Could a valve problem cause the engine to throw a lean code? how about an intermittent miss?
I thought I would ask these questions before just slapping parts on. I recently watched my brother spend 3 grand on someone else to throw parts at a “bank 2 detects lean code.” And, the problem still isn’t resolved. Could valves be his problem too?
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any engine should be well above 69psi. that engine shouldn’t even run that low! are you sure you tested it correctly? with a good fully charged battery?
ash? as in the plugs are black ashy looking like oil is being burnt? or as in over fueled? the valves would normally seep slowly when not running and you would just burn oil on startup and clean up after running awhile. sounds to me like it is just worn out. i don’t know the milage on the engine but hard running and poor maintence will eat up a good engine fast. also there is the chance of a bad oil filter once that had a holy threw the filter part it self.
if the plugs are black and ashy from to much fuel and the computer says it is lean i would head to the 02 sensor. I have yet to find a full proof way of testing a 02 sensor other then just replacing it. or them? i am not sure how many your car has off the top of my head.
that was my concern… to tell you the truth, this is the first compression test I’ve ever done. I’m pretty sure I did it right. I’m not sure if the valves are adjustable or not. Never fiddled with the valves on an OHC engine. I will look into that and try to figure that out. It’s not underpowered or anything, but you certainly don’t get the get up and go like that of a mustang GT. I always chalked it up the the tiny gears. Now I’m not so sure.
The battery is brand new, and pushing 14 volts. The O2 sensors were the next stop, but I don’t want to dump any more money into this car if the engine is toast. I may do it anyway. It only has one upstream and one downstream, and they’re fairly cheap (as long as you don’t get motorcraft.)
The plug looked just like the one in the back of a Hayne’s manual described as “ashy”. Fairly normal with a little bridge of carbonized ash under the electrode. The exhaust smells rich and kinda’ sweet, and occasionally spits bluish white smoke.
As far as the vehicle history is concerned. It has 80k miles on it, but it sat neglected for a very long time (guessing 4-7 years). I bought it not knowing this but figured it out after I started to tear into it.
Once again, I’m pretty darn sure that I did the compression test correctly, but I will watch Eric’s video again and try it again in the morning, just to make sure.
The 4.6L engines from that vintage had a poorly designed intake manifold, which was prone to leaking. There’s an updated part that is plastic and aluminum, rather than being purely plastic. It’s possible that you’ve got a cracked intake manifold or a worn-out IM gasket. Of course, you’ll need a fresh gasket if you replace the manifold as well.
For the lean code. I would look for vacuum leaks.
use a spray bottle filled with water. next check the
mechanical timing. you could also do a power balance
test. heres some videos to helphttp://www.youtube.com/watch?v=9CPqbaSg … plpp_video vacuum
http://www.youtube.com/watch?v=bAOmUjAj … plpp_video power balance
http://www.youtube.com/watch?v=X_tbksFY … plpp_video compression
I thought there might be something off with my rental compression tester, so I tested my pickup (07 F150 also a 4.6L), and it only read 80 psi. And, that truck runs like a champ. The tester I’m using is just off.
i knew something was not right about them numbers. still go on with the vacum leak check said above and if no luck there i would try some 02 sensor’s
Power balance seems normal. got a pretty good drop in RPM’s on every coil. Triple checked for vacuum leaks with a propane torch, and came up with nothing. I think I’m going to go to the parts store and get some new O2 sensors.
As far as the air intake manifold, I would believe it. That was on my list after the O2 sensors. I was going to take it off and put on new gaskets, and physically clean the injectors and put on new O-rings. I live in the desert and rubber rots quick here.
Would bad O2 sensors cause it to run rough?
02 sensor tell the computer how rich or lean the engine is running. if they are telling the computer you are lean and your not it will add fuel witch could be to much and make it run rough. and it can go the other way saying you are rich and really be to lean.
also when checking for vacum leaks i see eric did not touch on would be heater controls and vacum booster for your brakes. if you have a vacum operated heater controls you could have a leak inside the car and cannot find it from under the hood. same with the brake booster. i normally just pinch off them 2 vacum lines close to the intake and see if it change’s anything. BTW just a spray bottle with water works to for checking vacum leaks.
no I wasn’t holding the throttle open. OOPS! might have to go rent that thing and try it again.
What I did do is replace the upstream O2 sensors. I didn’t do the downstream because I couldn’t locate enough of them locally, and the code was related to the upstream. Naturally the code went away when I disconnected the battery, I drove it for about 20 miles around town and a little on the highway, and the light didn’t come back on, but it’s still running as rough as ever. How long does it normally take for the computer to catch up to a new part?
Also, is there a way to check your mechanical timing without taking it down to the timing chain? I’m sure it’s about time for it to be changed anyway, so if I have to do that I will.
humm maybe you had one problem then the 02 sensor just finally came up. most of the time it would catch up right away. how about a simple tuneup? have you done that since the car had sat? something is strang here and i am kinda drawing a blank. are you sure you did the power balance test right? sorry i don’t mean to be rude and 2nd guess your work but something just don’t make sense. did you try the pinching off of the brake booster and heater control vacum lines and see if that helped?
i hope some one els can see what i am missing here.
It’s not really “going through” any water, but I do have a bit of a cooling problem. My temp gauge will go up to about 2/3rd’s of the way up before the cooling fan kicks on. Then it goes back to halfway. Thought it was the thermostat so I replaced that. Didn’t solve it though. I just chalked it up to the way it worked on this car, but maybe that’s related some how.
The oil: It seems to be between 1/4 and 1/2 of a quart low when it’s time for an oil change.
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