Bucking under Acceleration, not fuel pump/filter

modulation · Post by **modulation** » Thu Aug 20, 2009 1:56 pm

S13 Ka24de

One day when I tried to drive my car, when I give it 20-30% throttle it starts bucking and refuses to accelerate. It had hiccuped a couple of times now and then a few days before but now it just refuses to accelerate.

I have a wideband and I can see that it leans out really bad >16:1-20:1 and up when I try to accelerate so it's fuel not spark.

I have a diagnostic scanner so I know it isn't:
MAF, TPS. CAS, Temp sensor.

I got done replacing the original fuel pump today with a Walbro and a new fuel filter but the same problem is going on.

Any ideas? Don't bad FPR's normally cause a car to run rich?

TM_S13 · Post by **TM_S13** » Thu Aug 20, 2009 6:44 pm

it depends... I did saw an OEM FPR giving problems, making the car run lean when the throttle was depressed fast, and when changing gear...

Thevlguy · Post by **Thevlguy** » Thu Aug 20, 2009 7:37 pm

a missfire will read a lean condition due to oxygen still being present, so dont rule spark out completely

modulation · Post by **modulation** » Fri Aug 21, 2009 4:21 pm

TM_S13 wrote:it depends... I did saw an OEM FPR giving problems, making the car run lean when the throttle was depressed fast, and when changing gear...

I read some where else that some one's failed like that too.
That gives me hope.

I got a new JECS FPR today for $99, haven't had time to put it in but I'll let you guys know how it goes.

modulation · Post by **modulation** » Fri Aug 21, 2009 4:24 pm

Thevlguy wrote:a missfire will read a lean condition due to oxygen still being present, so dont rule spark out completely

Cool I didn't know that.
I'm pretty sure it's not spark though since it only does it under load, and the higher the load the worse the bog.

It also seems like the higher the fuel pressure would need to be (to the right and up on the map) the leaner it runs.

It also smells lean and my friends watch when I try to accel and don't see anything black smoke or anything coming out of my tail-pipe.

I'm gonna replace the FPR and if that doesn't work then get a fuel pressure gauge and verify it is fuel pressure that's the problem.

Adam · Post by **Adam** » Fri Aug 21, 2009 4:43 pm

Remember higher the load, higher the boost, you could be flaming out you plugs, check plug gaps etc, spark could still be the issue, don't rule it out...

modulation · Post by **modulation** » Fri Aug 21, 2009 4:55 pm

Adam wrote:Remember higher the load, higher the boost, you could be flaming out you plugs, check plug gaps etc, spark could still be the issue, don't rule it out...

Roger. I'm NA so it's not as large of as difference but yeah I won't forget spark promise.

I did replace 3/4 spark plugs with new ones. Gapped them to the minimum gap spec. The old ones were only a few months old though. (Didn't do the 4th cause my strut bar is in the way, will do it this weekend.)

The plugs looked to be running lean, and maybe I just wanna think this, but the 4th one, the one closest to the FPR end of fuel rail seems to be running the leanest.

modulation · Post by **modulation** » Sun Aug 23, 2009 11:19 am

Well it isn't fuel.
I clamped the return line and I'm having the same problems.
Friend and I checked out spark and everything is cool.

Engine is running really lean now and doesn't even like to idle.
We are pretty sure there is a massive vacuum link in the intake area some where, but we've sprayed carb cleaner everywhere and can't get the idle to raise.

The problem has gotten worse..

We pulled the fuel line and it's spraying perfectly and everything.
All the injectors resistance test out well, and after pulling numerous spark plugs it still doesn't seem to just be one cylinder..

kenbam · Post by **kenbam** » Sun Aug 23, 2009 7:35 pm

Just because it hasnt logged a code for the AFM, doesnt mean its not causing the problem. I had a very similar problem with my rb20, sprayed some carb clean over the hot wire and it was as good as gold.

modulation · Post by **modulation** » Mon Aug 24, 2009 2:28 am

kenbam wrote:Just because it hasnt logged a code for the AFM, doesnt mean its not causing the problem. I had a very similar problem with my rb20, sprayed some carb clean over the hot wire and it was as good as gold.

I swapped out to a different MAF and I'm still having these problems.

modulation · Post by **modulation** » Mon Aug 24, 2009 3:28 am

Well I found some coolant on the floor this morning where there shouldn't be any. Its on the side of the engine with no coolant lines at all.

I'm thinking headgasket, got a friend coming over with a compression tester soon.

modulation · Post by **modulation** » Mon Aug 24, 2009 7:31 am

Did a compression test today.
All cylinders are within spec and no real compression difference between then.

I'm thinking a intake manifold gasket is leaking.
The intake manifold is 2 parts on my car so there are two gaskets that could be leaking.

Car idles fine when cold, but when it warms up it runs worse and worse.
If I touch the gas pedal when the car is warming up and idling ok it'll start having problems and not return to a normal idle.
If you let the car cool down then start it again it'll run find until it warms up.

Not the CTS I've checked that numerous times.

modulation · Post by **modulation** » Mon Aug 31, 2009 11:37 am

I was out of town at a track day at Thunderhill (in my VW R32 since my 240sx is down.) When I got back, my fuel pressure gauge was waiting for me.

I got a new battery today, and all the proper fittings to hook up the fuel pressure gauge to the line. Fuel pressure checked out, 44 PSI. Removed plugs one by one, and put them against strut tower. Tried to start engine and all of them are getting spark. All plugs were fouled a bit and smelled like gasoline (probably because mixture isn't igniting.)

I'm going to check compression again but I've ordered the two intake manifold gaskets (got the thermal one to the head) and I guess I'll be doing a intake manifold gasket job.

I also have a coolant leak, and I'm worried that's coming from the timing cover (it's on the driver side of the engine.) The timing cover couldn't cause a vacuum leak could it?

modulation · Post by **modulation** » Sun Sep 06, 2009 12:01 pm

I finally got the manifold off today.
There were some spots on the intake-> head gasket that indeed looked like they had fluids all the way though the gasket, as well as the fact that the coolant part of the gasket came off in two pieces.

The lower intake manifold gasket also looked like it might have been leaking but I'm not sure about that. The top one I'm 90% sure had a good leak, and might have even been letting coolant into the intake manifold. I'll find out when I get all the gaskets in, and put everything back together, probably late next week.

modulation · Post by **modulation** » Mon Sep 14, 2009 11:46 am

#1. I used a Fuel pressure gauge. Fuel Pressure is at factory Spec.
44PSI when fuel pump is primed, and 44PSI while cranking. Still won't start.

#2. I got extra intake manifold off craigslist. Blocked off all emissions stuff.
Use idle set screw to set idle. All new gasket, used torque wrench and FSM specs/directions to put intake manifold on.

Same problem as before I redid the intake manifold.