North American Turbocoupe Organization



kick and buck after 4k rpms
smallblockfloyd Offline
Junior Member
#1
Hey guys, I have been having an issue with the beast bucking and kicking past 4k rpm's at WOT/boost conditions. I first thought IAC or vac/boost leak. So, I replaced the IAC. helped the idle a bunch, but still persistantly bad at 4k+ rpms.

Since I run an aftermarket boost/vac gauge directly from the manifold and nothing else from the vacuum tree but factory boost gauge/brake booster i ran the brake booster directly to the upper manifold, removing the vacuum tree and possible leaks from the equation. This also improved the idle quality abit, and pushed my vacuum at 800 rpm from 8-10" mercury to 10-12" mercury. I am running an engle camshaft with 114* lobe seperation just FYI.

Do not have a wideband to watch the fuel ratios during boost. If I am cruising around town, and get the engine above 4k rpm's even without boost/WOT it will die when i let throttle go to idle position and no load on engine(ie clutching).

If I boost it, or even take it above 4k rpm's and then keep load on the engine it barely stays alive, and has no power for about two minutes. The plugs are looking rather black and nasty. This leads me to believe I am losing pressure from somewhere, or getting vacuum from somewhere after the air meter.

Running a NPR intercooler with ic piping similar to stingers, for stock location intake. Also, previous owner installed a kirban adjustable FPR. Possibly low/high fuel pressure causing poor conditions?

Also, these conditions have been persistant. I first thought my J3 chip(previous owner) was poorly tuned, so i pulled the 55# injectors and j3, returning the LA3 to stock tune, and i replaced the 35# injectors back to their rightful place. The problem did not change at all.

Anyone have a clue what could be the issue? Thanks guys!
84 SVO, holset, npr, 55# injectors, engle slider shaft, port work, 3.73 rear cogs.
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Pete D Offline
Administrator
#2
I don't know the specs on your cam but 10-12 seems kind of low.

How's fuel supply and pressure?

Not sure on this but is the vacuum booster for the brakes suppose to see boost?
Pete Dunham


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smallblockfloyd Offline
Junior Member
#3
Fuel supply is great. walbro intank 255 hp-lph. 'jumping' the fuel pump via test connector and not having the line connected to the rail, i can shoot a steady stream of fuel ten-fifteen foot across the parking lot.

Now, the pressure at the rail, not so sure. Kirban Adjustable FPR, previous installed and 'set up'. Haven't gotten a 1/16th npt 100psi gauge to test it with yet.

The vacuum booster was hooked up to the vac tree, same as the boost gauge. It has a one way valve that will allow vacuum to 'boost' the brakes, but positive-vacuum will close the valve, preventing power brake boost loss. It works just fine. And, i typically am not braking while boosting.

I too was thinking 10-12 seemed low for the shaft. What could cause low vacuum? Engine was claimed to have been recently rebuilt, less than 1500 on it. The previous owner also put a 1035 felpro gasket on it, and when i got the car, the gasket was blown. I got to do a hg swap and it was very fresh looking in the cyls. Fresh cross-hatching, pistons had no slop in them. Head was ported out a bit too. All of it looks good.

It really seems like unmetered air to me. Any thoughts? The previous owner also removed the PCV system and stuffed K&N breathers in the valve cover and the block. I don't quite think this would cause a significant problem thou.

Cam specs: Lift Int .442, Exh .434, [email protected] Int .270, Exh .262, Lobe Lift Int .276 Exh .271 Lobe Seperation 114*.

It should have very little vac. I have spoken with some other people with that camshaft, and they said they typically had 14-16" mercury at ~800rpm idle. I am just a bit curious what else I need to find/fix. Any thoughts?
84 SVO, holset, npr, 55# injectors, engle slider shaft, port work, 3.73 rear cogs.
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Pete D Offline
Administrator
#4
Stock idle rpms is suppose to be around 1K. This will probably improve you vacuum at idle also but I don't know how much.

Sounds like you need to put a guage on the rail and run the fuel system diagnostics.
Pete Dunham


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smallblockfloyd Offline
Junior Member
#5
Yea. the idle is typically around 800-900 rpms. Will open up the idle screw a bit more, but that doesn't address the current problem.

If fuel pressure is to high, what would this cause? Would it also cause hard warm starts? If I run the engine for more than a minute, then shut it off, wait 5 seconds, then go to start it again, it struggles. I have to let the starter hit, then go WOT to prevent injectors from flooding engine. this shouldn't be. either way, it sounds like too little or too much fuel pressure to me, causing poor injection mist.

Idle pressure without vacuum/boost assist is 39-41psi correct?
84 SVO, holset, npr, 55# injectors, engle slider shaft, port work, 3.73 rear cogs.
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Pete D Offline
Administrator
#6
Quote:Idle pressure without vacuum/boost assist is 39-41psi correct?
The acceptable range is a little wider but some consider that ideal. That assumes everything else is correct, like injector sizing and J3 tune and no air/vacuum leaks and lots of other stuff.

You don't set the idle by just turning the screw. The TPS also has to be adjusted. It really is important to follow the procedure below. See
http://www.turbotbird.com/techinfo/Idle-...e2005.html
Pete Dunham


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Ricky03 Offline
Senior Member
#7
Mine did the same thing an I could never figure it out. Have you had the battery unhooked for a while? I had been working on something but I can't remember what and had the battery disconnected for some time, about a month. It would miss at 3500 rpms every time. After driving it for about a tank of gas it went away never to return.
88TC :5 speed swap,255 walbo fuel pump, 3" down pipe with 2 1/2"duals, Ported upper and lower intake, ported E-6, Kiben adjustable fuel pressure regulator,T-3 turbo,Forge BPV,Steeda Tri-x shifter,Autometer gauges,Racer Walsh stage #2 cam, Spec stage 3 clutch,Stinger FMIC
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smallblockfloyd Offline
Junior Member
#8
It does it from 4k to about 6k. after 6k it quits 'missing' but the plugs get fouled and it wants to die, due to poor combustion. The TPS is within range of .92-.98(?). I last checked it being ~.94 volts at 'idle' position.

I've put a good two tanks thru it now, and it stays that way. I pulled all the j3 stuff out. it is just running on 35# injectors(brown tops) and stock la3 tune. am running hte proper sized vam aswell. Everything should be ok. I need to check the fuel pressure, and reset the idle a bit. either way, I think it is a vacuum leak issue. I just can't figure out where to peg it to yet. I may be wrong, and it is just a compounding issue of poor fuel pressure and tps/idle adjustment. but it should be in spec. will check tonight and see.
84 SVO, holset, npr, 55# injectors, engle slider shaft, port work, 3.73 rear cogs.
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Dave8338 Offline
Senior Member
#9
You've mentioned a vac leak but from what you are describing you're in boost as things start to go south. This being the case it is unlikely that a vac leak would have any effect and were it bad enough...you would see it in the lower RPM's more so than at the top end. Also a vac leak should lean out the air/fuel ratio (white plugs)
If I'm throwing my hat in the ring...I'de check your fuel pressure at the rail first and do this with the gauge positioned so that you can read it while you're driving.
If the plugs are balck and fouled it is suggesting either way too much fuel or an oil blow-by condition past the rings, as your pcv has been removed. Your Kirban may be set too high.
'86 TC: Granatelli adjustible clutch kit, Energy Suspensions, they own me, New KYB's, NOS heater core in the back seat, Chuck Warren Motor Mounts, Wallbro 255, Kirban FPR, New aluminum dual core. The Goal...all I need is the TIME!!
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4cammer72 Offline
Posting Freak
#10
lifters pumping up could also cause this problem hanging the valves open.
88tc (sold) but keeping an eye out.
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