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Car bucking under load

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15+ Year Contributor
Nov 19, 2006
Greensboro, North_Carolina
I finished reassembling my car and got some issues. First off the car takes about 6-8 tries before it will start up cold. next the car fights revving up. while i drive it, if i try to keep it at a steady rpm its fine, then the second i give it even the slightest bit of gas it starts bucking heavily. this is where i'm at now.

this is where i was at before:
When i first got the car running it ran like i stated above and the check engine light was on. sometimes i would drive the car and it would build rpms fine, only the boost didnt kick in till way late and the power wasnt there. this reeked of boost leak to me. so first i pulled the CE code and it read knock sensor. i checked the voltage of the harness side of the KS and it read 12 volts which to my understanding is normal. next i did a boost leak test and found a MASSIVE leak at the j pipe. i figured i was home free. i fixed the leak and took the car for a run. the boost kicked in a little sooner and with a little more power but still no punch. again, this was when the car decided not to buck on me. the CE light came back on. this time the code says malfunctioning ecm (manuals term). now im lost

the way the car takes several tries to start and bucks under load makes me think that my coilpack can be bad, but all 4 plugs have spark. Maybe i have a bad injector or something. at this point ill take anything you guys have for me.
the car idles fine. it does backfire a little bit though at an idle
Have you checked your timing?
And also, judging by the backfire and bucking..that makes me think about your Air to Fuel ratio.. You might want to check your injectors.
Also, try running down to AutoZone. Pick up a can of Berryman B-12. It's a fuel additive and will help to clean your injectors. I really think that would help.

Let me know how it goes man.

Also, do you have a CEL for your O2 sensor? It might show up as a fuel trim code.
A bad front O2 sensor could effect the way for car runs.
yeah check your timing. Also check your fuel pressure if you can. Being that its hard to start im leaning towards something messed up on the fuel side. Fuel pressure regulator vac hose maybe?
I had this problem when my timing belt was off a few teeth. My tensioner failed and let the belt slip which in turn bent my intake valves. Also check your maf and do a boost leak test. A clogged fuel filter could also cause this problem.

Good Luck
It could be a misfire which typically happens under load. Pull plugs and make sure they are gapped correcly (you'll also get an idea if you are running way rich by examining plugs) and make sure the plug wires are in good shape. If those check out then test your coils.
i am adding to this since i helped work on the car. heres the dilema, we pulled the engine apart to do all the major mechanical stuff, timing belt, tentioners, water pump, etc. he had the head fitted with new valves, had the engine timed. now we are working out the kinks. it has new wires. new ngk spark plugs gapped at the proper gap, cant think of it off the top of my head. when cold, the engine rotates 6-7 times without a hint of it catching, then it starts. when cold it runs decent. misses a little at idle, but when you hold the rpms at like 2300 it misses. so with him holding the throttle, i advanced, retarted the timing without any improvement. so i figure, clogged fuel injector, carbon checked/cracked distributer cap, bad sensor. what do you guys think. im new to the turbo world, only working on two of my own turbo cars. this is the first dsm i have even been under the hood of. the other thing is, e is running a straight down pipe with no exhaust hooked to it. i know a turbo creates back pressure, but could a lack of exhaust add to it?
heres the update: the boost leak test showed no leaks, i tested all 4 injectors and they work fine, all 4 plugs have spark, timing is dead on, MAF is working properly.

I pulled the CE code again and it read 31, knock sensor. This can explain the lack of power, but it doesnt explain the difficulty with cold starts. Before i go out and spend $70 on a knock sensor, does anyone know of anything that might throw a knock sensor off, or can i believe the code 100% that the sensor is bad? And as far as the cold starts go, the symptoms point to a bad coil pack (hard cold starts, bogging under load) but all 4 plugs have spark. is there another way to test the pack or can i assume that that needs to be replaced as well?
I have the same problem. My logger picked up two codes cylinder 4 misfire and wastegate malfunction?
I have the same problem. My logger picked up two codes cylinder 4 misfire and wastegate malfunction?

I would start by checking the spark plug and plug wire for cylinder 4. as far as the wastegate situation goes are you boost creeping? what exactly are you experiancing, a bogging (the car jerking around) or a sudden and violent slam like you hit a brick wall?

(wow, im seriously helping on my own help thread:rolleyes:)
When my coil pack went bad It had the same starting,backfiring at idle,and bucking problems. at first I could barely notice it,but it gradually increased.
^^^ yeah this is what im leaning towards, but at $100 to replace i wanna make sure i rule everything else out
Yea i experience boost creep with my T25 and once i hit 5000rpms on first gear it doesnt want to boost.Yea it boogs.
Well I really hope you find the problem,I forgot to mention the plugs 1&4 (companion cylinders) were oil fouled.
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