My '88 2.5l Fiero mostly runs and drives great, but I've been struggling for a while with a very rough idle misfire that causes the idle speed to "hunt" between about 500 rpm and 1500 rpm. This sometimes even causes the engine to fully stall out, particularly when it's entered closed loop control but is not yet fully up to temp. I don't want to go too deeply here into everything I've already tested/replaced (it's just about everything at this point), but particularly want to ask about a strange condition I've noticed that seems to pretty much exactly match what this guy posted about a long time ago:
Long story short, I pulled out my PCV valve with the engine running last week and noticed that a TON of hot gas seems to be entering my crankcase - enough that it feels like the PCV grommet on the valve cover is a second exhaust pipe when the valve is removed. Assuming that this is exhaust, the PCV is basically acting like a second EGR valve at all times!
I assumed that this hot gas in the crankcase was blowby and that my rings were probably shot, so I ran a compression test today. Results were 145, 150, 145, and 155 psi - so, nothing too out of the ordinary.
My thought at this point is that there must be some crack/leak/etc. in the head/valvetrain that's letting a ton of exhaust into my crankcase right under the valve cover. However, before I go fully rebuild my head, I wanted to ask two questions:
1. The loads of exhaust gas coming out of my valve cover isn't normal, right? My first assumption was that this is definitely a problematic running condition, but I wanted to make sure I'm not missing something here and that this isn't something that just happens to every duke.
2. If there is something seriously wrong here, what's the most likely culprit? Do I likely have a crack somewhere in the head, or could this be as simple as a leaky valve seal?
Some other notes: -I don't have any of the classic head gasket failure signs. No oil in coolant, no coolant in oil, no sign of exhaust bubbles in coolant, and no white smoke in my exhaust. -I hardly lose any oil - maybe a half a quart every 1000 mi, and I never see any blue smoke in my exhaust.
Thanks in advance for the help!
[This message has been edited by sbauco (edited 06-10-2023).]
Put the PCV back in and remove the oil fill cap. If you have a lot of air exiting the valve cover at the oil cap, then you have blow by.
If you have blow by, then your air filter housing will also be coated with an oil film on the inside. If you have blow by, then you likely need to clean the IAC port as well as the throttle blade in the throttle body. The oil muck is likely building up on both and restricting air flow through both and causing the stalling issue.
Blow by is typically a ring/cylinder wear issue.
[This message has been edited by fieroguru (edited 06-10-2023).]
Put the PCV back in and remove the oil fill cap. If you have a lot of air exiting the valve cover at the oil cap, then you have blow by.
If you have blow by, then your air filter housing will also be coated with an oil film on the inside. If you have blow by, then you likely need to clean the IAC port as well as the throttle blade in the throttle body. The oil muck is likely building up on both and restricting air flow through both and causing the stalling issue.
Blow by is typically a ring/cylinder wear issue.
I do have a very slight oily film right by the breather on the air filter housing, but nothing significant. Blowby was my first thought, but if my rings were that worn then shouldn't I see that in my compression test results? This is why I feel like there has to be something going on within the head/valvetrain that's letting the exhaust in, but I'm not sure where to start looking.
As for the IAC/throttle body, I cleaned and rebuilt the whole throttle body with new gaskets and seals a few weeks ago - unfortunately, no change to idle condition.
Blowby was my first thought, but if my rings were that worn then shouldn't I see that in my compression test results? This is why I feel like there has to be something going on within the head/valvetrain...
I believe you have it backwards. Healthy valves will result in good compression. But if the cylinders and/or the rings are worn, some of that good compression will be forced past the rings.
Clean/Fix all grounds and battery cables in engine bay and all power to "+ box" under C500. Check power at battery engine off and running. Have battery tested using computer testing not only simple load test. Have alt tested.
------------------ Dr. Ian Malcolm: Yeah, but your scientists were so preoccupied with whether or not they could, they didn't stop to think if they should. (Jurassic Park)
Originally posted by sbauco: I do have a very slight oily film right by the breather on the air filter housing, but nothing significant. Blowby was my first thought, but if my rings were that worn then shouldn't I see that in my compression test results? This is why I feel like there has to be something going on within the head/valvetrain that's letting the exhaust in, but I'm not sure where to start looking.
As for the IAC/throttle body, I cleaned and rebuilt the whole throttle body with new gaskets and seals a few weeks ago - unfortunately, no change to idle condition.
If you don't have a lot of oil residue inside the filter housing, then you likely do not have an excessive amount of blow by. The oil in the air filter would mean that the blow by is exceeding the amount of air that is pulled via vacuum through the PCV valve into the throttle body, so instead of air being sucked in from the air filter housing tube, it is being pushed back out of the engine and into the air filter box area.
Excess oil on the rings can help mask worn rings and scored cylinder walls during a compression check.
For the issue to be a head or valve train issue, the crack or leak would have to make it around the water jacket areas to a crankcase/valve cover area. A head gasket leak in a very specific spot could do this, which would mean 1 cylinder has an issue. An exhaust valve guide & seal wear issue could also do this to some degree, but would also result in puffs of smoke from the exhaust during cold starts.
If is was my car, I would do a leak down test. If one cylinder is much worse than the others, then that is where the issue is and likely means it needs a teardown to fully understand what is causing the issue. If the leak down test comes back clean, then there likely isn't any issue and what you are seeing with the PCV system is normal. https://www.mobil.com/en/lu...o-do-a-leakdown-test
"Blow by" problems isn't just for iffy piston rings...
Iffy Valves and Head can seal the valves for compression but Valve Stems, Guides and Seals can the Intake side to suck oil, Exhaust Side to cause "Blow By" problems or Both.
I had to Replace the Valve Seals recently because the "Rubber" got hard and 1 exhaust seal broke a "spring" that clamp the seal to head. That seal was "floating" on the stem above the head doing nothing.
While Can replace the Seals in the car but when the Stems and Guide wear too bad then replacing the seals won't help much if at all.
Your valve guides and/or seals could be worn. Do you see smoke on a cold start? If you suspect that your valve seals are getting hard you could put some AT-205 in your engine to see if that softens up the seals.
Thanks for all the advice everyone, and sorry for the huge pause in reply! I couldn't work on the car for a few weeks, but I finally got a chance to give it some attention this weekend.
So first off, I think I'm a bit of an idiot. After being stumped by what I thought was blowby, I pulled my PCV again and ran the engine to try and replicate what I initially felt. When I put my hand up to the PCV hole, I quickly realized that what I thought was loads of exhaust gas coming out the port was actually just reciprocating positive and negative pressure - i.e., I also felt my hand getting sucked into the hole. At higher RPMs, I didn't feel anything coming out of the hole. So, at the end of the day, I think that I don't have any blowby issues and that I was just grasping for straws to explain my weird idle, haha.
Next, I took theogre's advice and cleaned up all the grounds and positive wires in the engine bay per his cave. Unfortunately, this had no effect on the idle. My voltage while running is 14.1V solid according to my ALDL reader, so I'm doubtful that my alternator has any issues, but could there be some voltage ripple that I'm just not seeing on my code reader? I'm not sure if this is a common problem with these cars.
After that, I took a look at fuel pressure since I've never exonerated that either. Solid ~11-13 psi while running, so that seems unlikely.
Finally, I think I made some progress after replacing the MAP sensor and its connector. I replaced the MAP a while ago with a crappy Duralast one, but I then stumbled across this thread and decided to pick up a Standard brand MAP sensor instead. My idle immediately improved, and the hunting condition is basically entirely gone now. I did also get an intermittent MAP low code, but quickly realized that my connector was damaged, and this problem went away after I soldered in a new one.
So finally, I have a new question: how good of an idle can I reasonably expect? The car no longer hunts between RPMs, but I do still notice a good deal of idle misfires occuring - probably 1-2 per second. My BLM is also a bit high at idle - it sits at around 153 warm. Is this normal? I live in California where I need this thing to pass smog, so I need it to basically run perfectly. For reference, here's what I've already done:
-Replaced TPS, MAT, IAC, MAP, EGR, O2, plugs, wires, coils, ICM, ECM, EPROM (with new ATBS chip), fuel pump, fuel filter, and fuel injector -Rebuilt throttle body with new gaskets and seals, as well as a new FPR -Capped all vacuum lines to confirm no leaks, including brake booster line
At this point, I think just about the only things that could be wrong are that I could have a small leak somewhere in the intake manifold gasket, I could have a wonky alternator that doesn't provide a stable voltage, or I could have a poor quality rebuilt injector. However, the big question is: should I try and get the idle better, or is a slightly misfire-y stable idle about as good as it gets for a duke?
Thanks again for all the help!
[This message has been edited by sbauco (edited 07-26-2023).]
Plugs are basic copper AC Delco, R43TS6, with out-of-the-box stock gap.
When I bought the car it was actually missing the ICM shield, but thanks to your cave I knew I needed one! I didn't know where to find one, so I fabbed up my own out of steel.
...the big question is: should I try and get the idle better, or is a slightly misfire-y stable idle about as good as it gets for a duke?
My first Fiero back in the mid 1990's was an '87 duke, basically the same engine as the '88 minus the balance shafts and weird oil filter location. I hated that engine. As you know, there's no way to change the ignition timing, and it's so freakin' advanced at idle. I had nothing but trouble getting that car to pass the smog tests. Did I mention I hated that engine?
hey how come nobody has suggested changing out the PCV itself as the can spoil a perfect tune all by themselves. they can get crudded up or the spring gets weak or broken. people don't realize the pcv actually sends a metered amount of crankcase gasses/air back to the throttle body to be burnt off and if the are not working properly can throw off your tune especially idle.
My first Fiero back in the mid 1990's was an '87 duke, basically the same engine as the '88 minus the balance shafts and weird oil filter location. I hated that engine. As you know, there's no way to change the ignition timing, and it's so freakin' advanced at idle. I had nothing but trouble getting that car to pass the smog tests. Did I mention I hated that engine?
^This is exactly what it feels like! It feels like the timing is so advanced at idle that it's constantly on the edge of missing. Despite the idle RPMs being fairly stable now, the idle exhaust note has that airy, sort of lean sound to it, and the misfire rate is oddly high.
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Originally posted by sleek fiero:
hey how come nobody has suggested changing out the PCV itself as the can spoil a perfect tune all by themselves. they can get crudded up or the spring gets weak or broken. people don't realize the pcv actually sends a metered amount of crankcase gasses/air back to the throttle body to be burnt off and if the are not working properly can throw off your tune especially idle.
This is a good point; a new one is literally $2, so maybe I'll toss one in and see what happens. My current valve is pulling vacuum properly, but I could imagine that maybe the flow rate is reduced if there's a bunch of crap inside the valve.
Oh also, there's another weird symptom that I forgot to mention. Ever since I bought the car, the oil pressure gauge has been super wonky. It reads pressure properly at idle, but for some reason the gauge tends to randomly peg at maximum when I'm driving. Oddly it seems to especially like to to this when I'm off-throttle. I always assumed this was just a weird issue with the oil pressure sensor, but could this be indicative of broader electrical problems that could also be affecting the idle?
It reads pressure properly at idle, but for some reason the gauge tends to randomly peg at maximum when I'm driving.
The oil pressure gauge will peg if the sending unit is disconnected. Check the connector for looseness.
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Originally posted by sbauco:
It feels like the timing is so advanced at idle that it's constantly on the edge of missing.
Just for "fun", put a timing light on there and see how far advanced the ignition timing is while idling. It's no wonder that engine won't idle smoothly.
