Leaning out in low rpm's - WHY??

[jb89coupe]

Ok, here's the story. See the cardomain link in my sig for a full rundown of my setup. I recently did a H/I swap, and ever since my car has been hesitant, bucking, and sometimes backfiring (intake) at low rpms (until about 3k). It's somewhat inconsistant.. sometimes if you give it some gas down at the bottom of the gear it will start to move, then choke up and start sputtering, then all of a sudden around 3k it will snap back to life like a switch was flipped and plant your head back in the seat. IT RUNS GREAT AND PULLS LIKE A BEAST IN THE UPPER RPMS.

So far I have.. checked and triple checked the plugs and wires, replaced the TFI and PIP units (actually swapped the whole dizzy with a known working one), played with timing and fuel pressure. The o2 sensors are brand new, the ACT and ECT sensors were brought over from the old lower. The whole TB assembly was on the car originally complete w/ TPS, EGR, and IAB.. TPS was reset to .98v and checked for a smooth sweep.

I'm running out of ideas. If it were a fuel delivery issue (insufficient fuel), why would it run great at higher rpm's when the motor needs the most fuel? I've done a cylinder balance test (result 99 everything ok), the car is not throwing any codes..

These parts that were installed at the same time:
Heads
Intake
AFPR
Injectors

Everything else was on the car prior to the swap and it ran fine. I would imagine again that if the regulator or injectors were bad the problems would show up in the upper rpm range when the car needed more fuel, not down low..

Anyone have an idea?? I'm getting really frustrated here..

 

[vristang]

Are you sure the MAF is correctly calibrated to the new injectors? How does the motor respond to throttle position. For example WOT at 4k vs part throttle at 2k.

I think you are right, if it were fuel delivery you would probably have trouble at high RPM as well. This leaves vac leaks, emissions, and fuel ratio devices?

 

[jb89coupe]

Are you sure the MAF is correctly calibrated to the new injectors? How does the motor respond to throttle position. For example WOT at 4k vs part throttle at 2k.

I think you are right, if it were fuel delivery you would probably have trouble at high RPM as well. This leaves vac leaks, emissions, and fuel ratio devices?

Yes the MAF is calibrated for 19's, and worked fine with my stock injectors before the swap.

Once the motor is out of the "bad zone" (idle to around 3k rpm), it drives fine and is very responsive. At the low end, even at part throttle you can feel it lacking power, and if you get on it it will buck and stutter until it reaches around 3k. You can feel it spike and lunge out sometimes in the low rpms but it just chokes and stumbles immediately after. In other words I know the power is there, something is just holding it back..

I have no emissions equipment on the car.. the smog holes on the back of the heads are plugged, the vac lines that went to the air pump valves are capped.

The o2 sensors are brand new bosch units, not sure what else is an a/f device on these cars..

I was under the impression that a vacuum leak would result in a rich condition at lower rpms.. the fuel pressure is decreased as the manifold vacuum is increased, correct?

 

[1993SSP]

Ive had the similar problem too. My car is like it misses when you lug the car around taking it easy, floor it and it will feel sluggish, sometimes backfiring and missing, I too have done everything I can think of. Im thinkin my problem is my mass air meter, I swapped to the 24lb injectors, ran them with the 19lb maf till my 24maf came in, then i noticed the problems. Any way to check a MAF??

 

[vristang]

Confusion???
Are you still running the EGR? Your first post mentions it.

 

[gt90stang]

jb89coupe,

Mine did that after going to 42# injectors and a KB Blowzilla w/ 8 psi. I had to tune the accel enrichment fuel table and add 7# per minute to the lower 4 throttle angle rates. It was going lean on acceleration. You need to monitor your motor with a WB02 and real time data logging to figure out what is happening, otherwise your just guessing. I use an LM-1 and Tweecer RT.

Good Luck, Don

 

[mpswirl]

kinda sounds like the egr is sticking open a little bit.

 

[strtracer442]

Check for vacuum leaks. A vacuum leak will cause a lean condition at low rpm, at higher rpm's the leak is a lower percentage of overall air intake, there fore the mixture will be closer to being correct, so the problem seems to go away.

 

[jb89coupe]

Thanks for the ideas.. yes the car still has an EGR valve, it's a Ford replacement and worked fine before the swap.

So I guess I should be looking for a vacuum leak? I'll have to pick up a gauge I guess, I've just been tracing the vacuum lines everywhere and making sure they're all connected/capped. What should I get for a reading at idle?

 

[HISSIN50]

a stockish motor should pull close to 20" hg. with your mods, ill let others answer, but i would expect at least double digit numbers.

how is the brake booster doing? do you have hard brakes after a braking?

good luck.

 

[jb89coupe]

a stockish motor should pull close to 20" hg. with your mods, ill let others answer, but i would expect at least double digit numbers.

how is the brake booster doing? do you have hard brakes after a braking?

good luck.

Brakes are fine.. the only thing I can think of is maybe the lower didn't seat right. The heads are milled .040" so there was a pretty big gap at the end seals.. I used the cork gaskets that came with the set and a thick bead of rtv on either side. Seemed alright, and I've tried spraying carb cleaner around the ends to see if it would bubble or effect the idle - it hasn't. Anyone have a vacuum diagram, maybe I'm missing something?

 

[jb89coupe]

Well, I'm getting 18.5" at idle.. when reved it jumps to 0 then shoots right back to 18.5.. seems normal and I think I can rule out a vacuum leak.

So, it's not vacuum.. not ignition.. not fuel...

??? ???

 

[gt90stang]

jb89coupe,

When did you disconnect all the smog stuff? The PCM expects the EGR to be there and will throw codes if it isn't working right. It may even go into limp mode (bad), not sure on this. However, the PCM adds timing and messes with the fuel delivery when it is using the EGR. Check for codes and work from there. The EGR gets shutdown at WOT anyways so it doesn't really cost any power.

Good Luck, Don

 

[jb89coupe]

EGR is still there sorry for the confusion. No air pump, lines are capped, heads are plugged. The same problem exists at WOT too.

OK, if I unplug the vacuum line going from the manifold to the fp regulator, that should keep the regulator from reducing fuel pressure under vacuum, right? I tried that, and capped both ends to make sure it wasn't leaking, and I'm still leaning out down low.

I think it's a sensor/computer issue at this point, I've pretty much ruled out everything else. Like I said the o2 sensors are brand new, and everything else was in the car before so it's known working.. What else should I try??

 

[jb89coupe]

bump..

 

[vristang]

When I broke the EGR intake pipe on my 2.3L it ran like crap, even at WOT. I assumed the computer was throwing me into limp home mode. Since you have done some work on the car recently it may be worth looking over the pipes, especially around the welds.

 

[UMDSmith]

Mine had an identical problem when I removed the charcoal canister and unpugged that sensor that goes to it. The second I plugged that sensor/valve back in, it worked as normal again. I'm at work so don't know the exact name for what that sensor/valve is. When it was unplugged, my car would buck, and sputter at low rpm, then smooth out a bit at high rpm or under a decent amount more throttle.

 

[jb89coupe]

When I broke the EGR intake pipe on my 2.3L it ran like crap, even at WOT. I assumed the computer was throwing me into limp home mode. Since you have done some work on the car recently it may be worth looking over the pipes, especially around the welds.

I'm not sure what/where the EGR intake pipe is?? The only things going to the EGR valve are a vacuum line and the electrical connector for the sensor.

 

[jb89coupe]

Mine had an identical problem when I removed the charcoal canister and unpugged that sensor that goes to it. The second I plugged that sensor/valve back in, it worked as normal again. I'm at work so don't know the exact name for what that sensor/valve is. When it was unplugged, my car would buck, and sputter at low rpm, then smooth out a bit at high rpm or under a decent amount more throttle.

I did have the vacuum line that goes from the canister to the manifold off for a while, but I hooked it back up the other day.. no difference. I'm going to try swapping my stock injectors and regulator back this week and see if that does the trick, I'm running out of things it could be.

 

[1993SSP]

I spoke with the guy who is gonna do my dyno, he told me this problem could be due to a grouning problem, told me to double check all my grounds including the one from the head to the firewall and to closely check all my O2 sensor connections and harness