Georgia Fiero Club Forum
All Things Fiero => Tech Tips, Tech Questions => Topic started by: f85gtron on November 05, 2017, 06:59:38 am
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I'm trying to diagnose a rough idle on an 84 duke (which also had bogging on throttle tip-in). I fitted a fuel pressure tester to it last night (it was dark out), and fired it up to check running pressure. Well, I noticed light coming from the intake cover wing nut holes and took a look through the space. There was flame coming up through the iac and getting sucked down into the intake. I could smell fuel burning and I shut it down. I've smelled that smell before, so I know of one other occasion it's happened. I've never seen this before. What causes it? How do I correct it?
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Just to be clear: flames were coming from the IAC? It's basically a stepper motor with a rod on it.
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Yes, flames where coming up or down THROUGH the iac, so does that mean there's too much fuel in the intake manifold? How did it get ignited?
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That rough idle may be due to a burned intake valve that's igniting fuel in the intake? But it seems that if that were the case, the whole intake system would explode or the fire wouldn't go out when the engine is shut down.
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I've had my second cup of coffee and am thinking a little better now. I'm not really familiar with the 4 cylinder, but I know the EGR valve is sitting right in front of the throttle body. Is there any possibility that it might be defective and allowing exhaust gases to enter the intake?
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I don't know about the L4, but the gases that come out of the EGR to the intake on the V6 are not hot. If the EGR is releasing hot gases to the intake, it'd be easy to test.
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I guess i could block it off to test. Also, looks like a compression test is in order! I might try the compression test first.
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Burned or sticking intake valve or bad EGR valve could both cause bad running.
(Are the exhaust valves usually the ones that burn?)
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You could have a leaking injector or injector o-ring. Disconnect the wire from the injector while the car is running, and see if it stops immediately, or runs on for a bit.
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I don't know about the L4, but the gases that come out of the EGR to the intake on the V6 are not hot. If the EGR is releasing hot gases to the intake, it'd be easy to test.
Are you sure? The V6 EGR tube is wrapped in a heat blanket from the factory. Rodney's replacement has a tube shield around it.
The L4 allows exhaust gases to pass directly into the intake manifold when it's open, being as there's no tube. I just checked in a Chilton's manual and it has a cross-section diagram of the EGR gas flow.
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Well, I thought it was hot for exactly those reasons, but I grabbed it while the engine was hot and running, and it wasn't hot. That may be symptomatic of an EGR issue, but I've had several people tell me that the tube doesn't get hot. I have no idea how hot exhaust gas could be cool.
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After reading this (http://www.fiero.nl/forum/Archives/Archive-000001/HTML/20120111-2-106052.html), I believe my EGR is not working, and that the tube should be hot, at least 178 degrees.
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It probably won't get hot if just sitting and idling since the EGR system only works at road speeds, then it pulls hot gases into the intake.
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You could take the EGR off, make a plate out of a piece of aluminum from a drink can, put it over the hole, and put the EGR back on. The see how the engine idles.
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If like the Old school egr's you can slip a couple fingers into the diaphram (from the underside of the Valve and raise it up) thus stalling it at Idle or Hold a finger against it and raise the RPM to road speed and feel it raise up like it should if operating correctly. If it doesn't stall at Idle it's plugged up and needs to have the port Cleaned or the Valve replaced.
if not raising up above idle the same will be true. We used a speedo cable cut about a foot long chucked up in a drill motor to spin it and ream out the port. use the end that has a square on it where it fits into the gear or speedo to chuck up into drill motor.
these ports get a carbon build up in them.
caution they DO get HOT
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Ok. Back on topic.
I checked the ecm coolant sensor. It showed 3100 ohms at 70°f. That's good, right? The spray pattern is even and no drips. Key on prime also no drips.
I'll do a compression test maybe this weekend and report results.
Any other suggestions?
Could timing be too advanced? 8° on number 1 for duke, right?
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The GM Temperature sensor chart shows 3400 Ohms at 70* F. But at 90 degrees, it drops to 2150, so 3100 should be close enough.
Looking at some manuals, there is exhaust gas ported directly through the EGR valve pintle seal and into the intake. Since the exhaust manifold is on the opposite side of the head, there has to be a direct port from probably the number 2 and 3 cylinder exhaust valves, over to the intake. A diagram of the L4 head appears to support this, as it shows a port that's definitely not one of the fuel intake ports to the head.
If that EGR valve is stuck open, and you have a misadjusted or burned exhaust valve on one of those cylinders, it may be possible to ignite fuel vapors through that passage. Just a theory on that.
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Compression:
I performed a compression test.
Dry. Didn't do a wet, I was satisfied with the dry.
#1@120, #2@115, #3@140, #4@120. The only suspect would be #3. The rest seem close to normal.
Fuel:
Fuel pressure stayed at 12# running and didn't fluctuate much during revving.
Key on runs pressure up quickly, but falls quickly. No change when return is clamped. Also, tbi is dry during test, no leaks from injector.
I haven't checked the egr yet.
The fuel pressure leaks down so fast, i find it suspect, but stays up when running, so I don't know what to think.
Any thoughts?
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Could it be that the fuel pump is holding the pressure up when the engine's running? Leakdown as you described could be a bad regulator, or is it possible that the check valve in the filter is bad and letting pressure bleed backwards through the filter?
There are warnings about removing the screws for the regulator diaphragm cover due to the heavy spring underneath. Also about fuel spray if the pressure isn't relieved before opening up the regulator.
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I rebuilt the tbi complete with new regulator. That doesn't mean I didn't screw it up though.
Here's a video of it's behavior.
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Question:
On the v6, the pressure rises when the fuel is goosed. Shouldn't the tbi do the same? I did notice it's not vacuum controlled at all. Just a spring, so I guess I shouldn't expect it to. I just would like someone's experience on that part.
Observation:
I hooked up tuner pro to see what the ecm sees and it had a value of 195.7° for the coolant temp. It is about 60° in the garage and I don't think anyone else was driving it... That leads me to believe that the coolant temp sensor needs replaced. So I guess the 3100 ohm value wasn't good enough for gm. Oh well. Back to the drawing board.
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Question:
On the v6, the pressure rises when the fuel is goosed.
Goosed? Pressing the accelerator? The only reason I can think of for that to happen is that the alternator turns faster, making more power for the fuel pump. All things considered, the pressure should otherwise go down.
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On the v6, there's a vacuum on the spring side that holds the regulator back. The more vacuum (idle), the less fuel pressure. As the throttle is goosed, vacuum drops and the spring does it's thing. The pressure then rises during that event to feed the injectors.
I don't see anything like that on the duke. It's a low pressure system and I'm not used to it.
The false hot coolant sensor reading explains the extended cranking to start and crappy cold characteristics. It's always in closed loop instead of open in the beginning, which means no choke, no cold crank fueling, and ecm calc based on a cold o2 sensor that would be far from accurate for a few seconds at least.
I still have to cap off the egr to test. I agree that the exhaust side is the only way that the intake could have ignited while running.
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What TopNotch said was what I was thinking, too. Most V6's run at about 38-40 psi at road speed. In absence of a vacuum function, the Duke regulator has to be an equalizing pressure type regulator where the pressure drops on one side will cause the valve to open to try to attain equal pressure on each side of the diaphragm. I used this type regulator extensively in water pressure control on golf courses. Whe n pressure dropped downstream due to increased demand, the spring pushed the valve open to equalize demand and supply side pressures. Very efficient with miniscule lag time.
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So it's normal then?... to run at 12# with minimal fluctuation? Sounds like you're saying yes. From what you're describing, sounds like it's good to go.
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I don't know for sure, Ron, but that's the way an equalizing valve works. Think of the pressure regulator on your home water system. It works the same way. Water pressure drops at the faucet due to demand and the valve opens up to try to equalize the pressures on the up and downstream sides. The thing I don't understand is how the injector pulse widgh is actually increased, unless it's through signals from the throttle position sensor. The pulse width would need to be increased as speed increases, essentially the same as opening the faucet more.
Our V6's have an over-pressure return line, but does the L4? The pump needs fuel moving through it any time it's running in order to keep it cool so it would need a return bypass line.
I didn't have time to fully read this, but maybe it will help. Headed out the door.
https://www.hemmings.com/magazine/hcc/2011/08/Half-Breed/3702631.html
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Does your car have the little tin "deflector" under the throttle body, in the intake manifold?
Many people have speculated regarding it's intended purpose. Some suggesting that, whatever it is, it restricts air flow.
Perhaps it's there to divert EGR gases away from the throttle body opening. (Assuming it's missing.)
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Our V6's have an over-pressure return line, but does the L4?
Yes.
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I think it's there. I have to look again. I forgot about that deflector.
Yes, the 4 banger has a fuel return line too.
The pulse width of the injector calc is derived from several variables. Mostly from the map vs rpm vs fuel. It's a 3d table that if load (map), and rpm, then fuel specified.
The tps only is used for pump shot (acceleration enrichment), entering wot (power enrichment), decel derichment, and delta movements to change spark at diverting rates if using rpm derivative spark.
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Any update?
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I replaced the cts last night and it's running mucho better now. Revs up nicely, no flaming. The only thing i can figure is it was too lean because the temp was reading too high and the flames spread easily up through the intake. I'm just guessing at this point.
Next, I've got to bleed brakes and give it a run to get the air control valve to learn 0 and idle right.
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Your brakes are bled properly when you stand on the brake pedal from 70mph, come to a complete stop and, the 'Spatula Option' comes out of the dash and scrapes you off of the inside of the windshield. ;D
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Um, those aren't Fiero brakes. They may be on a Fiero, but that I've never had a Fiero stop like that. In a 'Cuda dragster, letting off the throttle slows the car down faster than the stock brakes in a Fiero.
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You must not have an 88. The stock brakes are good on 88 Fieros.
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Not good, just better. I have an '88. I improved my brakes, and now they feel modern.
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Blocked off egr. Revs up normal now, but now runs rich at idle for a little bit. Chugs and belchs black smoke until warm.
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Did you block off both ends of the EGR tube, disconnect the battery for a few minutes and then run it?
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I forgot to reset the ecm. Duh!
I blocked off with a coke can at the gallery. It's a duke. (No tube)
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tbi should have tipped me off. Oops. Best wishes for a 13.7 to 1 mix and no black smoke and belching.