Hey, long time listener first time poster. I've got an 87 SE w/ the iron duke and automatic 3 speed. Been having this issue since last fall where when I come to a stop then take off again, it'll move about 1 foot then bog down/fall on its face for a split second. If I anticipate it and try to give it more gas it'll just bog down harder. I can also sometimes tell it's about to happen because the idle feels weird (not bogging down, just, not right). Id say it does this on about 75% of take offs. As soon as the engine warms up the problem goes away, and apart from that it drives perfectly. I'm sure it's a vacuum leak somewhere with my immediate thought being a sticky EGR (which i did find wasn't bolted down tight and had a gap, but that fixed nothing) but I've tested the EGR and it's fine. Also no check engine for low vacuum.

Honestly just wondering if anyone else has had a similar issue or advice before I go through pulling off every vacuum and fuel line just to find it's something totally unrelated like a bad ground.

The first question that must be asked.....

Why would you put the engine under load until it warms up?

ALWAYS let the engine warm up before driving, especially older engines.
MOST wear happens to engines that are not warmed up before putting under load.

Hey another Chicago fiero! I wouldn’t be surprised if it was just a combination of the age of the engine, how many miles are on it, how well it’s been maintained, and these cold Chicago winters. Gotta remember these little guys are coming up on 40 years, which is like 200 in car years. My 122k mile duke does some weird stuff when she’s really cold too. Erratic and inconsistent idle, hard, jerking shifts (automatic), and generally just not being happy about the whole ordeal. Runs like a real sweetheart once she’s warm though. Sorry if this sounds like a cop out answer but it’s just my two cents.

quote Originally posted by olejoedad: The first question that must be asked..... Why would you put the engine under load until it warms up? ALWAYS let the engine warm up before driving, especially older engines. MOST wear happens to engines that are not warmed up before putting under load.

I don't fully subscribe to that. In winter, yes it gets a good 5 min to warm up. But it's a nice spring day, and it's a push rod engine. If the valves are not coated in the 2 min that it takes me to get settled and pick music then we have much bigger issues than a vacuum leak. Regardless, it will still bog down even if it warms up in the drive way. It's not until I open it up on the highway that it fully goes away. Also some days are better than others.

quote Originally posted by JigglyWiblets: Hey, long time listener first time poster. I've got an 87 SE w/ the iron duke and automatic 3 speed. Been having this issue since last fall where when I come to a stop then take off again, it'll move about 1 foot then bog down/fall on its face for a split second. If I anticipate it and try to give it more gas it'll just bog down harder. I can also sometimes tell it's about to happen because the idle feels weird (not bogging down, just, not right). Id say it does this on about 75% of take offs. As soon as the engine warms up the problem goes away, and apart from that it drives perfectly. I'm sure it's a vacuum leak somewhere with my immediate thought being a sticky EGR (which i did find wasn't bolted down tight and had a gap, but that fixed nothing) but I've tested the EGR and it's fine. Also no check engine for low vacuum. Honestly just wondering if anyone else has had a similar issue or advice before I go through pulling off every vacuum and fuel line just to find it's something totally unrelated like a bad ground.

Stumbling acceleration is usually as a result of too much or too little fuel. WHY that is, ends up being the big question. Especially since you have a DIS ignition, so we can largely rule out ignition as a failure here.

When the engine is cold, it usually needs more (than less) fuel, and so there's a number of things that go on with the Iron Duke to get it to warm up more quickly. The Fiero has a system called a ThermAC, which tries to restrict the air going into the engine, and replace it with warmer air. This does two things, one, it helps warm the engine up more quickly (by drawing air past the exhuast manifold), and two, reduces the amount of air going into the engine and thus requiring less fuel. Think of the ThermAC almost like an automatic choke on the engine. I don't know how old you are, but back in the day (really, before my time because I got my Driver's License in 1994 when I was 16), cars came with what was called a choke. It was a knob you'd pull out that allowed you to start the car when cold, and then as it warmed up, you pushed the choke back in and the car ran normal. At some point in the late 70s and early 80s, they replaced this with an electric choke which was a heated coil that, when powered, would retract and slowly pull the choke back.

The ThermAC is a very similar type of system. If it's inoperable, as in, the vacuum line going to it is disconnected, or the heater / vent tube is missing, then it's basically doing nothing, and your car will be getting too much air compared with the fuel being provided.

There's some other stuff that goes on too with the car's programming. On a cold start, the RPMs should be almost racing (like 2,000 rpms)... which I hate, but that's to get the motor warmed up as quickly as possible, and immediately it starts activing the thermac which constantly adjusts the air inlet valve based on a little blue sensor that measures the temperature. There's some other programming that goes into it... adjusting the IAC valve, and also the EGR. My daughter is restoring her car, and I like to post this video because it shows the normal cold start process of the 1984-1986 Pontiac Fiero 2m4. It should be nearly identical to your car, though you have DIS ignition and a newer style of TBI unit... but basically it's still the same, as in, you still have a ThermAC, etc.:





What I'll add about this is... in modern vehicles, they don't really need all that stuff. There's improved sensors, like the use of a MAF (Mass Air Flow) sensor, and an "Intake Air Temperature" sensor, as well as a better learning PID process, so modern ECMs can more appropriately adjust for fuel requirements no matter what the temperature of the engine is (cold start or not).

quote Originally posted by 82-T/A [At Work]: Stumbling acceleration is usually as a result of too much or too little fuel. WHY that is, ends up being the big question. Especially since you have a DIS ignition, so we can largely rule out ignition as a failure here. When the engine is cold, it usually needs more (than less) fuel, and so there's a number of things that go on with the Iron Duke to get it to warm up more quickly. The Fiero has a system called a ThermAC, which tries to restrict the air going into the engine, and replace it with warmer air. This does two things, one, it helps warm the engine up more quickly (by drawing air past the exhuast manifold), and two, reduces the amount of air going into the engine and thus requiring less fuel. Think of the ThermAC almost like an automatic choke on the engine. I don't know how old you are, but back in the day (really, before my time because I got my Driver's License in 1994 when I was 16), cars came with what was called a choke. It was a knob you'd pull out that allowed you to start the car when cold, and then as it warmed up, you pushed the choke back in and the car ran normal. At some point in the late 70s and early 80s, they replaced this with an electric choke which was a heated coil that, when powered, would retract and slowly pull the choke back. The ThermAC is a very similar type of system. If it's inoperable, as in, the vacuum line going to it is disconnected, or the heater / vent tube is missing, then it's basically doing nothing, and your car will be getting too much air compared with the fuel being provided. There's some other stuff that goes on too with the car's programming. On a cold start, the RPMs should be almost racing (like 2,000 rpms)... which I hate, but that's to get the motor warmed up as quickly as possible, and immediately it starts activing the thermac which constantly adjusts the air inlet valve based on a little blue sensor that measures the temperature. There's some other programming that goes into it... adjusting the IAC valve, and also the EGR. My daughter is restoring her car, and I like to post this video because it shows the normal cold start process of the 1984-1986 Pontiac Fiero 2m4. It should be nearly identical to your car, though you have DIS ignition and a newer style of TBI unit... but basically it's still the same, as in, you still have a ThermAC, etc.: What I'll add about this is... in modern vehicles, they don't really need all that stuff. There's improved sensors, like the use of a MAF (Mass Air Flow) sensor, and an "Intake Air Temperature" sensor, as well as a better learning PID process, so modern ECMs can more appropriately adjust for fuel requirements no matter what the temperature of the engine is (cold start or not).

Where would I find this vacuum line? I've tested the PCV valve and EGR already. Is it the other tube coming out of the valve cover to the air filter that isn't the PCV? I see you mentioned exhaust manifold but I've got nothing going on back there except the starter.

quote Originally posted by JigglyWiblets: I don't fully subscribe to that. In winter, yes it gets a good 5 min to warm up. But it's a nice spring day, and it's a push rod engine. If the valves are not coated in the 2 min that it takes me to get settled and pick music then we have much bigger issues than a vacuum leak. Regardless, it will still bog down even if it warms up in the drive way. It's not until I open it up on the highway that it fully goes away. Also some days are better than others.

Subscribe to it or not, I don't care. Always warm up the car.

You probably have other issues as well with the vehicle, but my advice on warming is sound advice.

quote Originally posted by 82-T/A [At Work]: The Fiero has a system called a ThermAC, which tries to restrict the air going into the engine, and replace it with warmer air.

There is no ThermAC system on an '87 duke. There is an Intake Air Temperature sensor instead.

Check for trouble codes.

A failing Intake Air Temperature and/or Coolant Temperature Sensor could both affect engine performance, as it's possible the engine is running too lean when cold.

A failing fuel pump can also lead to a lean condition, as can a leaking "rubber" hose in the gas tank. Testing the fuel pressure would be a good idea.

Although this was for my '84 duke, much of it still applies to an '87... Tried three different MAP sensors, but 2.5 engine still bogs badly when accelerated

[This message has been edited by Patrick (edited 03-25-2025).]

quote Originally posted by Patrick: There is no ThermAC system on an '87 duke. There is an Intake Air Temperature sensor instead. Check for trouble codes. A failing Intake Air Temperature and/or Coolant Temperature Sensor could both affect engine performance, as it's possible the engine is running too lean when cold. A failing fuel pump can also lead to a lean condition, as can a leaking "rubber" hose in the gas tank. Testing the fuel pressure would be a good idea. Although this was for my '84 duke, much of it still applies to an '87... Tried three different MAP sensors, but 2.5 engine still bogs badly when accelerated

I had no idea! Good to know...