3.4DOHC Help! (Page 1/1)
fliphone JAN 13, 07:28 PM
Hey guys! I've got a few issues and possible solutions I would like to ping off of you all in regards to my recently running/driving LQ1 swap. I'll give a little background on the car and some basic information about it. It's a 88GT with a 96/97 LQ1 being ran by a 91-93 wiring harness/ECU for smog purposes, the wiring harness was made by Erik, it currently has about 250-300 miles on the clock since i got it running/driving, the EGR has been deleted, the battery has been moved to the front, and it's yet to get a full exhaust so it's currently a piece of 1' pipe pointing down with an 02 sensor.

Most of my current troubles revolve around idling and hot starts. If I go to start the car after it's been sitting long enough to be fully considered "cold", it'll fire right up sometimes on the first crank and sounds pretty damn choppy at idle. Something odd that may be useful info is that the RPMs will spike to 2500-3000 before slowly coming back down, with the issue being worse hot going up to 4k at times (more on that later). I've given it a few minutes to warm up every time I've started it and it seems fine when I start driving, but it seems that the minute the car switches to closed loop some semi-consistent problems will emerge.

1. When the vehicle is rolling and in neutral the RPMs will jump up and down ~250-300 rpm, often between about 1200 to 1500 rpms. But this isn't a consistant issue, it will happen about 50% of the time. It could be a vacuum leak but there are very few vacuum lines left, I've got the brake booster, Charcoal canister, MAP sensor, and the fuel pressure regulator.

2. When I come to a complete stop/speedo shows zero, the car will lower the RPMs as expected. But occasionally it will bog down to 400/500 RPM or in the worst case die completely. And again, this isn't always. Sometimes it'll come down nice and smooth and act perfect, sometimes I'm scared to even bring the car down to stop, sometimes it takes a blip of the throttle to keep the car alive. This one I don't have too many ideas to work from, I will test my clutch switch to see if it's working. I will also check my grounds in/around the engine bay, I'm not 100% confident in how I ran them from block -> frame and it may be a partial explanation as to why this is all sporadic without any standout conditions causing issues

3. This is the worst of them all. The car HATES warm/hot starts. I've only had one time where I cranked the car over warm/hot and it started completely normal. I have to treat the car like a cold carbonated small block and feather the throttle until it "catches itself". Sometimes it takes a couple tries but I've always been able to get it pretty quick until last night. Last night after helping my buddy with his MR2, I went to start the car and it wouldn't start. It would occasionally fire, but if i let it would just crank and crank and crank and crank. After resetting the ECU and some parking lot testing some sensors, the thing that finally got it started after 30 minutes of tinkering was when someone covered the "exhaust" with their foot and give the engine some back pressure. After that I ensured it stayed in gear and tried to keep the load on the engine. When it had load it ran normal and I didn't feel anything alarming, but the whole drive home the car really didn't want to idle. The second i pulled into the driveway and took it out of gear it idled like crap for 5-7 seconds then died. There are a few possibilities that come to mind. The same grounds as mentioned above could be in play here. The coilpacks I got with this motor were from a couple different cars and seem that they've seen better days, and the heat soak may be bringing out their age. The final immediate thing that comes to mind would be the back pressure, or lack there of. From what I understand 4 strokes don't need back pressure, but most of my hands on experience with this has been with lawnmower engines and such. So do car motors need some back pressure to startup and run?
I tried and the car fired right up this morning, I let it idle for about 10 seconds (it was very choppy) and shut it off.

Sorry for this one being so wordy, I just wanted to give as much info as I could. Thank you in advance for any help/advice/recommendations you guys have, last night really threw me for a loop

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Black 88 GT, a 19 year old here about 20 years to late
- Running(?) LQ1 Swap-

2M6DOHC JAN 15, 09:47 AM
Hello,


I have built up a n/a dohc that I am running on a standalone ecu, in which what has happened to me may or may not apply to you.


Are you running stock injectors or compatible injectors for the motor, it almost sounds like a fueling issue. Are you running cheap injectors? I ran into a problem with cheap injectors where you could not get consistency out of the motor. Especially at low and high RPMs, I ended up with some nice bosch injectors for the newer 5.0 Mustangs, but then again, I was able to adjust for the higher poundage and different dead times in the ecu.

On the vacuum note, I was chasing vacuum leaks for quite a while until I ripped out all the plastic fittings and replaced them with metal fittings and hose clamps. Also replaced the rubber caps with silicon caps since I am not running a charcoal canister and some of the accessories that hook up to the vacuum ports. Which may or may not apply to what you have done.

I also recommend checking the EGR block-off plate that you are using to close off the port on the plenum, it is possible that you may have air coming through there.

Here is a suggestion that could be helpful if you plan on doing anything with the motor later on (i.e., performance modifications or whatnot) I recommend getting a wideband 02 sensor and a gauge. I am running two sensors, one on each bank.

This would give you back live AFR readings, which may or may not help you to diagnose the rough-running problems.

For me, I plan to do performance upgrades in the future, and since the widebands integrate into my ECU it can adjust and make tuning the fuel tables so much easier. However, I do not know if this is something you want to do for your setup.