1JZ stumbling/hesitation ADVICE?

[afracer]

Hey guys, I just got back from being deployed over to Iraq. While I was gone my 1jzgte Cressida starting acting up on my wife. First the alternator crapped out, it was replaced but now the car has a pretty bad stumbling and hesitation under partial throttle and just off-idle. Feels to me like a lean spot because sometimes it will also backfire through the intake under acceleration too. If you are stopped and just mash the gas, it revs slower and stumbles and sometimes backfires through the intake...once revs climb a bit more, boost comes on and it pulls hard to red line like it should. If you are just cruising along lightly touching the gas it does the same stumbling/hesitation and backfiring through the intake. I put new plugs in, new Denso O2 sensor, battery, and checked all the connections out which all look fine. There's no vaccuum leaks I can find anywhere and the hose going to the MAP sensor is just fine. It seems to me like fuel isn't right...possibly timing, but I'm pretty certain I'm getting a lack of fuel at partial throttle. I think maybe a fuel injector might be going bad. Anyone have a good way to check the injectors out? I had a VERY similar problem with my SR20 when one of the injectors went bad...partial throttle sucked and hesitated everywhere but full throttle was fine. I will be swapping/upgrading the fuel pump today to a Denso 280lph pump from the Chaser pump it currently has. Keep in mind this car is NOT a recent swap...it's been swapped and running great for about two years now with absolutely no problems before the alternator went bad. It has stock twins, non-TRC throttle body, automatic, FMIC, 3" exhaust. If anyone has any advice or help, I'd be very grateful! I will be replacing the PCV valve and probably also throwing in my wideband O2 sensor, but it's not going to tell me anything I don't already know (lean spot) probably. If the fuel pump doesn't take care of anything which I doubt it will, then I might swap out the Idle Air Control Valve that someone recommended to me could be it, otherwise I might swap the injectors with some other stockers to see if that does anything but that doesn't look like a fun job. If none of the above works then I'll start thinking about sending the ECU to get new capacitors in it, but hopefully I'll chase this down before getting to that point since I don't have a spare A/T ECU anywhere. I wan to get it up and going before Thursday so we can take it on our road trip instead of our 240SX. Someone help please!!

[afracer]

Pics of my car since I can't post them on my Intro thread (too old).

http://img.photobucket.com/albums/v258/afracer/Chaser/Cressie2006_0906009.jpg
http://img.photobucket.com/albums/v258/afracer/Chaser/DSCF1414.jpg
http://img.photobucket.com/albums/v258/afracer/Chaser/sarah2402007_0401004.jpg
http://img.photobucket.com/albums/v258/afracer/Chaser/sarah2402007_0401007.jpg
http://img.photobucket.com/albums/v258/afracer/Chaser/sarah2402007_0401006.jpg

[Kosh]

Wow i am having this exact same problem, i was just coming on here to post this question. For me there is that hessitation like if i'm partially on the thottle and then let off a little the engine will kick off and engine brake hard and then without hitting the gas much more it'll all of a sudden kick back in. I think it might be because the throttle cable is getting stuck at that point. I notice that when i ease off the gas really slowly it transitions fine, but when i do it a lil quicker i get the problem.

Could this be the effect of having too long of a throttle cable? I think thats what it might be for me at least? comments?

[Freshmaker]

afracer, are you getting any codes? Its pretty easy to check the ECU for bad caps. I know a couple guys have recently posted pics on here with bad caps.

Kosh, check your dashpot and adjustment

[drifter7126]

damn...have the same problem...someone please shine some light here will ya!?

[drifter7126]

oh and i almost forgot...my ecu is good also..so i know its not the ecu that is causing the problem during this stumbling/hesitation.

[Kosh]

Kosh, check your dashpot and adjustment

:confused:

no idea how to do that or even wut that means...lol

[Freshmaker]

It is a little vacuum valve. It acts like a shock absorber. When you throttle snaps shut it leaves the throttle partially open for a bit right at the end. Kind of hard to explain, really easy to understand if you are looking at a throttle plate as it happens. On the 1jz throttle body, it is on the opposite side of the throttle cables. It has an adjustment screw, a short vacuum line and a vacuum shock-absorber-type-thing called a dashpot.

Once you locate it, open the TB to WOT and release it. It should slam into the dashpot and slowly close from about 5% to 0%

[Kosh]

It is a little vacuum valve. It acts like a shock absorber. When you throttle snaps shut it leaves the throttle partially open for a bit right at the end. Kind of hard to explain, really easy to understand if you are looking at a throttle plate as it happens. On the 1jz throttle body, it is on the opposite side of the throttle cables. It has an adjustment screw, a short vacuum line and a vacuum shock-absorber-type-thing called a dashpot.

Once you locate it, open the TB to WOT and release it. It should slam into the dashpot and slowly close from about 5% to 0%

awesome advice, from the sounds of what it does it sounds like it may solve my problem. Thats why u suggested it i guess :) lol

It's stalling on me now too...the harder i'm pulling the easier it stalls when i let off the gas. If i'm coming up to a light for example and i let off and hit the brakes and clutch it'll stall. And if i'm just driving really easy, then it drops off to like 10 RPM but will catch itself and go back up to normal idle. It sounds like this "dashpot" is not actuating fast enough or whatever? we'll see i guess.

[Kosh]

Once you locate it, open the TB to WOT and release it. It should slam into the dashpot and slowly close from about 5% to 0%

this is just to test it right? i'm assuming since its vacuum actuated that i have to test it when the car is running? what do i do then if it isn't working properly? How do adjust it properly? just play around with it till it works right? lol

[annoyingrob]

it's not vacuum operated on the 1JZ is it? I'm sure the 1JZ dashpot is a self-contained unit.

[Freshmaker]

Im sorry, it works with vacuum type valve, but it doesn't require the car to be running. If you look at it, its only connected to the outside of the TB. It never sees any of the engine vacuum. When you slam the throttle close it creates some pressurized air in the line,then the valve lets it out slowly. That gives it the shock absorber effect.

Yeah Kosh, that is the purpose of the dashpot. I pulled 5% out of my a$$ though.

[Kosh]

So just to get this straight. The car stalling on me would also be attributed to the dashpot right?

[Freshmaker]

Could be, either way its an easy check :P

[afracer]

Well, I don't know what the deal is but my car runs great now. I put the new battery in, new o2 sensor, plugs, tightened the alternator wire, and disconnected my fog lights that had a bad relay two nights ago. It still had all kinds of hesitation and stuff at that point. I woke up in the morning, started it up...it seemed like the idle air control valve wasn't working so good or something because it kinda choked itself off and died...rpms stayed real low after it started and just conked out, so I restarted, gave it a little gas to help it warm up and now the car runs just fine with no more stumbling. Whatever I did I guess fixed it, or something is just being finnicky or on it's way out perhaps... thanks for the thoughts guys, hope everything works for them too.

[afracer]

EDIT - Okay the car is NOT fixed. One day after it ran fine I started it up and it ran like crap again, backfiring all over the place and severe hesitation and what not. There was a bad coolant leak on one of the heater hoses that I fixed, and I swapped the ECU out with a good one and it still does the same thing.

So, tomorrow I'm going to do a little more searching around, check out the spark plugs, maybe try installing a new fuel pump and see if that does anything. My brother swears it's the pump, someone else swears it's the Idle Air Control Valve, so I'm going to check into those things tomorrow and see what happens. I've got a spare IACV to swap and I'm going to check the diagnostic codes and see what it might have in there now that I finally found out how to do it (I think).

If none of this works then I might look into new injectors or something, but I'm starting to run out of ideas. I miss driving my Toyota so bad, if you guys help me and I figure it out and fix it then I can finally save up and get ready to do the 5 speed swap.

So does anyone have any other ideas to help me out or have a similar experience??? Help if you can!!

[annoyingrob]

You sure the coolant's topped up? If it's low on coolant, the sensor will be sitting in air. Ask me how I know. haha.

[suprastanger507mgte]

HAve you checked the ignitor? It may start and idle and sound grreat, but when you give gas or drive it, it will crap out like you describing.

[drifter7126]

will the ignitor cause the stumbling in lower rpm and than as soon as boost comes on the car pulls good all the way to red line?

[afracer]

I haven't "checked" the ignitor because I only have one of those and its the one on the car. If it was, would the ECU throw a code for it? Anyone close to Abilene Texas have a spare ignitor?

[afracer]

I fixed the car today. I checked to see if the ECU would give me any codes and it didn't, I actually got a clean bill of health from it, which got me kinda suspicious about the fuel pump and finally decided to throw my new one in. I bought a new Nippon Denso 260lph 'Supra' fuel pump for it a few months ago because I wanted to upgrade for future power goals and because I really had no idea what kind of pump was already in the car from the previous owner (said it was a JZX81 pump I thought). Anyways, it was a direct drop in for the stock pump and after installing it, all my problems went away and the car ran awesome, like even better than it ever had before!

So I'm still going to send my stock ECU to BigAaron because when I took it out and opened it up there was definitely at least one leaky capacitor, so might as well take care of it now since the Cressida's dash is a nightmare to take apart, I don't want to do it twice. Thanks for everyone's help and advice, but I don't think anyone guessed "fuel pump" was the problem.