91 SC missing at low rpm's (I know I posted this before, but I never got any help!!!)

Bacondoggy

Registered User
I have a 1991 Thunderbird Supercoupe (less then 70k orig. miles). It was totally rebuilt. I know about tccoa, but there apparently s no one in this area. Long story short, I bought this SC (my second one) and had it for at least 4-5 years. the car had lower crank bearings in the bottom AND top (the oil couldn't get through the holes to the arm and pistons). I bought it this way (part of a piston was in the oil pan) You could still see the hone marks on the cylinder walls (bored +.030 over). We took the engine apart and rebuilt it from scratch (had the rods checked new pistons, rings, all bearings mounts, etc.). My dad and grandpa were VERY thorough to the point it annoyed me. Upon starting it, the car is missing VERY badly, at low RPM's. I replaced the IAC (new from ford - $120), I have 3 or 4 MAFS, Also 3 or 4 throttle bodies, along with TPS sensors. No Boost/Vac lak ... at least that I know, when it is idling, it is below the "ING-HNG" or whatever - (sorry I haven't started it in a LONG TIME - at least below -15/20lbs whatever is below "9 'oclock"). I am SO confused. Checked the fuel pressure at the rails, changed the fuel filter also.... I don't mind paying someone if they could help me out... I am just SO frustrated I don't know what to do...

ALSO: plugs/wires are new as well as O2 sensors...
 
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ALSO: plugs/wires are new as well as O2 sensors...

Coil pack wiring goes like this:

(front)
4-6-5
3-2-1
(back)

See if that helps you out any. :cool:

Oh, and of course.. reseal your IC tubes. Especially the joint at the intake plenum..
You can try to tighten them up first, but don't go too tight. Its best to remove the tubes
from the flanges, clean both mating surfaces and apply a suitable sealant. Copper RTV
works good, I used plain old black RTV and it works well just fine. You want a thin
smooth layer on both surfaces. Tighten evenly. Shouldn't need more than about 10lbs.
:cool:
 
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cam sensor?

I am sure about firing order and the wires... Also it is all sealed very well...I need to check the cam sensor though - It may be installed incorrectly. I remember my grandpa installing it with me, and he somehow used a voltmeter, and we installed it right when it started getting voltage as per the instructions he had. My question is couldn't it be 180 degrees off? he said that that shouldn't matter or something and I took his word for it because generally he is ALOT smarter then I am! But does anyone have the correct procedure for installing the cam sensor? Also, is there a way to tell if the sensor is BAD, or just mis-aligned?
 
Put the number 1 cylinder 26 degrees after top dead center on its compression stroke. If you take the cam sensor off, the little half moon should face the rear of the car at this point. When I was off a tooth on the distributor my car wouldn't start at all.
 
And you're absolutely certain of it?? ;)
Oh.. you can simply unplug the cam sensor.

yes I have the shop manual as well - and I hear you can unplug the cam sensor - but how will that help me with troubleshooting this? I don't have a problem starting my car (it starts every time). but I am now guessing I have a bad cam or crank sensor - but it seems like the cam sensor is either bad or 180 degrees off...
 
yes I have the shop manual as well - and I hear you can unplug the cam sensor - but how will that help me with troubleshooting this? I don't have a problem starting my car (it starts every time). but I am now guessing I have a bad cam or crank sensor - but it seems like the cam sensor is either bad or 180 degrees off...
If it runs good w/o the cam sensor connected, then you know that the cam sensor is causing a problem. If it runs the same, then you can eliminate the cam sensor as the problem. It will take a few tries to get it to start (key off between tries) but it will run. If your crank sensor is a problem, usually the car won't run at all.

Have you done a cylinder balance test?

.
.
I still think you may have a vacuum leak ;) ;) :D
 
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yes I have the shop manual as well - and I hear you can unplug the cam sensor - but how will that help me with troubleshooting this? I don't have a problem starting my car (it starts every time). but I am now guessing I have a bad cam or crank sensor - but it seems like the cam sensor is either bad or 180 degrees off...

The firing order in some manuals was wrong. I forget which ones it was right in and which ones it was wrong it, but it might be worth it to check what you have in your manual against what was posted above.

Sorry, other than that i really don't have anything to add to this discussion.

Does the condition worsen/improve/stay the same as the car heats up?
 
I haven't driven it recently, but what I can remember, I don't think so - its like 4000 rpm and under. And it is like I can't pull away from a stock 4 clyinder car! I don't get my license back till like march-august, and was trying to fix the car so I can drive it when I get it back (I lost my license for 1 1/2 years). I also posted the same first question on TCCOA, and I got alot of different and mixed replies (some saying how it can't be the cam sensor at all) you can find this link here:
http://forums.tccoa.com/showthread.php?p=1064090#post1064090

I am fairly sure there is no vac leak - all the ic tubes have been sealed with 1/4" of copper rtv (I also checked with carb cleaner, starting fluid, using soapy water, etc).

Also, what is a cylinder balance test?
 
also, could my FPR be bad? Could that be causing the computer to think its running lean? I remember there was god pressure at the rails, but I don't remember if it de-pressurized quickly... could this be why I am getting a lean reading for both banks, and the missing, but then when the car gets going (above 3500 rpm) it starts to pull boost and doesn't drive as badly? I would think if I had a fuel problem, I would have opposite problems (like under load not running right). Arrgh I want to flip out because I have been trying to troubleshoot this for years now...
 
If the FPR is bad you will more than likely be running rich as typically the diaphragm is ripped so it would be sucking fuel through the vacuum line. Pull the vacuum line there and smell it, strong gas smell = FUBAR'd FPR.

A cylinder balance test is done when you do what they call a KOER test with the car. Part of the procedure the computer does for testing is to shutoff fuel to each cylinder and measure the RPM drop, and if one is greater than the others then it will show a code indicating that cylinder with a problem.

Is the miss at idle erratic or is it fairly constant time apart?
 
ok - well I took my DIS and got it tested - it is fine - they ran the test 3 times to make sure it stayed good even when it got hot. Is the FPR beneath the intake plenum on the pass side? I remember smelling gas under the hood at one point in time - its been so long (1 1/2 years) since I really drove it, and that was the last time I did the KOER test. All I remember was 2 codes being thrown - engine running lean on bank a and bank b. I will have to check the idle again, but I remember putting on a different TPS and TB, and from what I can remember now it seems to idle normally, but still gets the CEL light randomly when driven.
 
The FPR is at the driver's side rear of the engine. It's sort of tucked in amidst the plenums and what not.
 
FYI...I had a 90 SC with wires on the cranks sensor that were stripped and were shorting themselves out. It would run after numerous attempts at starting it, but it ran like a b!tch. Just figured I would throw that out there....you never know.

Also....same car.....I replaced a tone of maintenance stuff on it when I got it. I checked the wires 5 times and thoght they were in the right order. Needless to say....I traced every wire by hand to make sure my eyes were seeing things properly and sure enough, they were wrong.
 
I had the same problem for a few months before I finally figured it out. The best thing for you to do is, get a code reader. They're like $30 from Advance Auto, and scan you're car for codes. READ THE MANUAL FOR THE SCANNER!!! You must read it, before you attempt anything.

Before I got my scanner, I was throwing numberous parts at the car, just hopeing one of them would fix it. It turned out to be a bad DIS Module. I got one off of here for $15, and haven't had the problem since.

Also, what you were told earlier about taking the Cam Sensor off, that isn't entirely true. I took mine off and the car ran really well! Replaced it with a new one, and it still ran very badly.

Just take you're time searching for the problem, and spend twice as much time searching for a reasonable solution.

Good Luck,
Dave
 
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