View Full Version : Need help diagnosing a miss.

Dustin Yenny
01-28-2005, 11:46 PM
I've had nothing but driveability problems with the 93 since I bought it. I mean, I love the car....just not the way it runs most of the time. I could not stand it and quit driving the car within a month of purchasing it.

To begin with the car had the wrong size injectors in it when I bought it. The PO thought they were 42# and could barely get the car to run as he had the MAFs calibrated for that size. I went back to stock injectors and MAF.

Now to the problem. Since I changed those items out, the car ran a little better. For instance, I took the car out tonight for the 1st time since October. I take off normally 5-8psi boost and the car runs fine. 30 seconds later I will try the same thing and it wil miss and buck absolutely terribly. If you felt it....I swear you would think the engine was gonna fall out onto the ground. No matter what, if I always take off slow and keep it under 5psi it will almost never miss or buck. Now lets say we are going down the road. I speed up and the boost passes 5psi. It does the same thing (one time it may miss and buck terribly, 30 seconds later when I try it again.....its fine).

I've cleaned the MAF, the plugs are Autolite Double Platinums, Magnecore wires, mostly every sensor on the engine is new including the cam sensor. I replaced both 02s due to a CE light. I've resealed the intercooler tubes numerous times. I was told the fuel filter was replaced.

I just find it hard to believe that its a fuel issue (like dirty injectors or a filter) as I would think it would be constant not very random. Fuel pump is a new Walbro 255lph. I've checked the pressure while driving the car and there is no problem there.

My thinking is its electrical like a bad DIS or coil pack. All I know is that if I could get it running like it should....I would keep it in a heartbeat. Actually, I guess I have no choice because I've been trying to cut my losses and sell it for a while and well its still here.

Dustin Yenny
01-29-2005, 01:31 AM
Here is some more information I feel is vital to the figuring out the problem. I was on my way down to the store to get something (its a small town I live in and mostly 25mph or 35mph). Anyway, I could not figure out why the car was being such a beast driving through town. I mean, I can take off and keep steady at ~1/2 throttle and I'm pulling 10psi and can not keep traction. Not to mention the car runs like it should.

After I went to the store and filled up with 93 (JFYI: I only run the high test in all my cars) I decided to do a little more testing.

It seems that the car runs fine if I'm under 40 and in either of the first two gears. This is appearant because I can romp it from a stop and besides going all over the road from the tires spinning....the car runs perfect until it shifts into 3rd gear when it starts the missing and bucking. If I slow down to say 25mph when I can give it enough gas to downshift into 2nd.....the car runs pefect and gets up to speed. Now, when it shifts into 3rd......anytime I go over 7psi boost......runs like crap. I tried this over and over again.....with the same results.

I do not understand any of this now.

01-29-2005, 08:25 AM
What was your fuel rail pressure?
What type (AOD or 4R70W) of transmission do you have?
When you changed the MAF sensor back, did you also change the sample tube?
Does the car have an aftermarket chip installed?
What size MAF and injectors are you using?

01-29-2005, 08:36 AM
Have you checked for codes?

XR7 Dave
01-29-2005, 11:06 AM
What was your fuel rail pressure?
What type (AOD or 4R70W) of transmission do you have?It's a 93 which means AOD.

When you changed the MAF sensor back, did you also change the sample tube?He said stock MAF, ie. no sample tube.

Does the car have an aftermarket chip installed?
What size MAF and injectors are you using? See above.

The car needs basic diagnosis. You start with the simple and eliminate possible sources until the problem presents itself.

Verify fuel pressure under load. If it's not rising lb for lb and holding then you have a fuel delivery problem. Most likely sources are fuel filter, crushed fuel line, bad fuel pump, bad regulator.

If fuel pressure is fine during symptoms, then the next likely source is plugs and wires. I've seen wires that won't stay on the plugs, cracked insulators, or excessive gap cause similar symptoms. Magnacore wires fit poorly, definitely try a stock set.

The next easiest thing is to swap known good DIS and coil (one at a time). These items can be tested, but sometimes with limited tools it is just easier to swap known good components.

I assume you are not getting any codes?

Good luck.

01-29-2005, 01:33 PM
[QUOTE=XR7 Dave]It's a 93 which means AOD.
He said stock MAF, ie. no sample tube.
See above.

Thank you Dave, you are correct. I just wanted more info. about what he was running now. Going from modified and back to stock, you find some pretty strange things.

Dustin Yenny
01-29-2005, 05:48 PM
Ok guys. Dave, thanks again for all your help. I changed the fuel filter earlier and that didn't help. I had pulled a plug to check the condition again...and they were indeed the APP2544s and looked new other than being a little white/grayish (which I was told was good). So, regardless I decided to change them. All Autozone had in stock were the APP764s so I put those in. Its nice having small arms/hands. I managed to get all the plugs replaced from the top in 45 minutes or less. Test drove the car and its running 100%. Maybe now someone will buy it. Its gonna be tough selling it now that it runs like it should....but I think I'm going to stick with my orignal plan to sell them all and use the money on a down payment for a new Subaru Legacy GT. I changed the oil, and wiper blades while I was at it.

Regardless, thanks for all the help and if anyone knows someone looking for a great car. Send them my way---$2500 obo. I transferred plates and insurance to it yesterday....so it is legal to test drive.

P.S. Dave, you still have that odometer with 114K? I'm thinking if I get that replaced since the car is just about at that mileage....it will be one less negative issue trying to sell the car???