misfire under load

[MOTORBREATH53]

I just dropped this stock 94GT long block in my 91LX.
Stock 302, Cobra Manifold, Cheap CAI, MSD Coil, T3550, MAC Shorties, Flowmasters, A9L
Fuel pressure is 42 at idle no vaccum
Plugs are autolite 25's set gap .050
Timing 10*BTDC

Idles at 700ish, revs well, and returns to idle well, but there is a slight mis at idle.
The real problem is driving under any load (ie any uphill, no matter how steep), I get a misfire that feels like a burnt plug boot. It hesitates, car jerks slightly, definatly losing power. Part throttle on flat ground feels ok...

Going down hill, I can stomp on it, and it feels ok?!

I have gone over every vaccum line with a propane bottle, and even pulled the upper back off and replaced the gasket...again.
I have tried two different dizzy's wtih two different rotors, tfi's
I have tried the stock MAF off my 94gt
I have tried pulling the plugs, regapping, they all look clean, equal etc....
I have tried two sets of plug wires on two different caps, along with misting the wires with water at night.... no spark leaking....
I have tried my MSD HVC Blaster coil, a stock coil, and even an old can style coil off my bronco.....
I checked the TPS and its at .80 closed. I cycled it through, and couldn't see anything I would call a dead spot...
The only code I pull is 44/94 regaurding the smog pump....

Any help would be.... well super....

 

[88mouse]

I'am having the same probablm i will be checking mine out tomarrow i will let u know what i find.

 

[HISSIN50]

Did you run a cylinder balance test yet? That's where I'd start since you noted the miss is felt at or near idle.

Good luck.

 

[MOTORBREATH53]

no luck

alright... so I tried to run the cylinder balance test, but the car dies at the end of the regular diagnostic test...
I could read the codes it was showing on my scan tool in the MIL, but then it would idle high as if still testing for approx 2.5-3 minutes. The idle would then just fall off and die.

what am I doing wrong.

 

[MOTORBREATH53]

losing my mind her gentlmen...
I did remove the 255 pump from the tank and replace it with the stock one from the 94.... maybe somthing is goofy and its running out of fuel... but if it were leaning out, wouldn't it pull a code from the o2's? or ping? And wouldn't the fuel pressure drop when reved?

any help guys... i'm about to just light it on fire....or at least use profanity....

 

[HISSIN50]

alright... so I tried to run the cylinder balance test, but the car dies at the end of the regular diagnostic test...
I could read the codes it was showing on my scan tool in the MIL, but then it would idle high as if still testing for approx 2.5-3 minutes. The idle would then just fall off and die.

what am I doing wrong.

When you start the CBT, IIRC you stab the throttle to WOT real quick. You can tell the puter 'took' the command because it will hold the idle around 1500 RPM (for fudge factor as it shuts cylinders down and an RPM drop occurs). It can take a little time for this to occur. You should hear the cylinders cutting out and back on.

Good luck.

 

[MOTORBREATH53]

Right, I understand that part, but when do I stab the throttle? After the codes have stopped flashing? before the idle falls off? How do I know that the regular testing is done?

And why would the car stall at the end of the testing?

Thanks for your help.

 

[jrichker]

Cylinder balance test:
Warm the car's engine up to normal operating temperature. Use a
jumper wire or paper clip to put the computer into test mode. Start
the engine and let it go through the normal diagnostic tests, then
quickly press the throttle to the floor. The engine RPM should exceed
2500 RPM's for a brief second. The engine RPM's will increase to about
1450-1600 RPM and hold steady. The engine will shut off power to each
injector, one at a time. When it has sequenced through all 8 injectors,
it will flash 9 for everything OK, or the number of the failing cylinder
such as 2 for cylinder #2. Quickly pressing the throttle again up to
2500 RPM’s will cause the test to re-run with smaller qualifying figures.
Do it a third time, and if the same cylinder shows up, the cylinder is
weak and isn’t putting out power like it should. See the Chilton’s Shop
manual for the complete test procedure

Here's the link to dump the computer codes with only a jumper wire
or paper clip and the check engine light, or test light or voltmeter.
I’ve used it for years, and it works great.

See http://www.troublecodes.net/Ford/
OR
See http://www.mustangworks.com/articles/electronics/eec-iv_codes.html

IF your car is an 86-88 stang, you'll have to use the test lamp or voltmeter method. There is no functional check engine light on the 86-88's except possibly the Cali Mass Air cars.

For those who are intimidated by all the wires & connections,
see http://www.actron.com/product_detail.php?pid=16153 for what a
typical hand scanner looks like. Normal retail price is about $30 or so
at Walmart.

Or for a nicer scanner see http://www.midwayautosupply.com/detailedproductdescription.asp?3829 – It has a 3 digit LCD display so that you don’t have to count flashes
or beeps.. Cost is $33.

Do a compression test on all the cylinders.
Take special note of any cylinder that shows up as weak in the cylinder
balance test. Low compression on one of these cylinders rules out the
injectors as being the most likely cause of the problem. Look at cylinders
that fail the cylinder balance test but have good compression. These
cylinders either have a bad injector, bad spark plug or spark plug wire.
Move the wire and then the spark plug to another cylinder and run the
cylinder balance test again. If it follows the moved wire or spark plug,
you have found the problem. If the same cylinder fails the test again,
the injector is bad. If different cylinders fail the cylinder balance test,
you have ignition problems or wiring problems in the 10 pin black &
white electrical connectors located by the EGR.

How to do a compression test:
Only use a compression tester with a screw in adapter for the spark
plug hole. The other type leaks too much to get an accurate reading.
Your local auto parts store may have a compression tester to rent.
If you do mechanic work on your own car on a regular basis, it would
be a good tool to add to your collection.

With the engine warmed up, remove all spark plugs and prop the
throttle wide open, crank the engine until it the gage reading stops
increasing. On a cold engine, it will be hard to tell what's good &
what's not. Some of the recent posts have numbers ranging from
140-170 psi. If the compression is low, squirt some oil in the cylinder
and do it again – if it comes up, the rings are worn. There should be
no more than 10% difference between cylinders. Use a blow down
leak test (puts compressed air inside cylinders) on cylinders that
have more than 10% difference.

See the link to my site for details on how to build your own blow
down type compression tester.


One of the tests with the engine running is the EGR function. It opens the EGR with the engine at or near idle. It is supposed to stumble or nearly die. Yours may be dying due to other problems that are happening when this test is run.

 

[MOTORBREATH53]

Thanks for all the help... you guys are awesome. Maybe if I try d/c the vaccume line to the egr before the test... it will throw a code, but maybe then I could try the test.
I'm beginnig to wonder if I have a vaccum leak on the lower somwhere... that would suck..... but even that would be better than not knowing anymore.

 

[MOTORBREATH53]

So, i disconnected the 02's with the car running.... it started to surge at idle, but I took it around the block anyway. NO MORE MISFIRE. So I took it up the big hill by my house and stopmed on it....NO MISFIRE, RAN GREAT! I LOVE PROGRESS! Now, is this showing that my 02 sensors are bad, or is it just running open loop and masking the problem?? This is definately a good step... but, what do you guys think?

THANKS AGAIN GENTLEMEN....:SNSign:

On a side note... I must be retarded because I tried the CBT 3 times, and couldn't get it to go. The test ended, I reved it to about 3k, breifly held it, and let off.... It would just return to idle.

 

[HISSIN50]

I actually cannot remember the time frame you have to initiate a CBT. I just always do it right after the codes are spit.

For no O2's, you're running in FMEM and it will default to a fairly decent tune.

You can try and watch your O2 signals to see how they're doing. They can become very hysteretic before a code is generated.

Good luck.

 

[MOTORBREATH53]

So, if the computer defaults to a good tune, and the car drives well, that should rule out spark delivery or vaccum leak issues.... right? It should also rule out clogged fuel pumps/filters etc. So....is the MAF bad? it is a junkyard part.... its clean and looks ok to me. Not to mention, I swapped the maf off my running 94 and no luck...

The 02s passed the resistance test. 3.4 and 3.6ohms warmed up.... I have some other 02s in my storage on my smog H pipe. Maybe i'll try those on.

 

[88mouse]

i just check my car for vac. leak everything was ok.I believe that its in the fuel systom. Mine started just after geting gas last week.

 

[HISSIN50]

I have some other 02s in my storage on my smog H pipe. Maybe i'll try those on.

This would be my next course of action. I didnt say anything before because O2's are pricey and we hate tossing parts at fixes, but when the parts are free, I have not one issue with trying it.

 

[MOTORBREATH53]

Well... I never made it over to get my other 02's... but the misfire is gone....

I reconnected my 02's, and cleared the KAMS. The only other change that I made was re-clocking the MAF, and I barely moved that so....
There is still a miss at ilde, the car drives great, and its not going to be mine anymore... let them fight with the idle.

Thanks for all your help guys... :SNSign: