When you click on links to various merchants on this site and make a purchase, this can result in this site earning a commission. Affiliate programs and affiliations include, but are not limited to, the eBay Partner Network.
My idle has been pretty inconsistent and I suspected an air leak somewhere. I smoke tested it and, sure enough, I found a pretty big gash in the throttle body boot. After fixing that and smoke testing again, the air leaks are gone. However, I'm still having some mild issues with my idle. It's an 87 CRX Si, so 1.5L I4 (D15A3) FI. Here's the behavior:
Once warmed up, idle goes up and down between 1200 and 1250 and sounds pretty smooth. It goes up to 1250, drops to 1200 in about half a second, then bumps back up to 1250 and repeats. Not that bad. HOWEVER, after between 8 and 12 cycles of going up and down, it will drop to 1000, the entire car will rattle for half a second, then it will jump back up to 1250 and repeat. So it's...
1250 down to 1200 in half a secondJump back up to 1250
Down to 1200 in half a secondJump back up to 1250
[Repeat 8-12x]
Drop to 1000 in half a second, car shakesJumps quickly back up to 1250
Back to beginning.
If it's not an air leak, what else could be causing this behavior? Any ideas are greatly appreciated!
Does your intake manifold and throttle body look like the one below? If so, I would be highly suspicious of the two vacuum actuators I've pointed to with the red arrows. I'm not very familiar with the pre-88 stuff, but I'm almost certain one of these actuators is for starting air. Here's how it works - there is a spring that holds the throttle cracked open during starting, then once the engine is running, the force of the vacuum on the diaphragm inside the actuator allows it to pull against the spring pressure and close the throttle plate fully. After that, the IACV takes over for controlling idle speed by bleeding air around the throttle plate. That's the intent of how it works, anyhow. The problem is that these cars are so old the diaphragm inside the actuator is almost certainly bad, meaning your throttle plate cannot fully close at idle because no amount of vacuum is going to pull the actuator to close the throttle plate against the spring pressure, and now you have a path for a vacuum leak. I had this issue on a B18A1 engine and was able to remove the actuator, plug the vacuum nipple on the intake manifold and it fixed my issue. You could try temporarily removing the actuator and plugging the vacuum line on your car to see if it works. If you have difficulty starting, you may need to crack the throttle (very slightly) with the gas pedal while cranking.
Also, just FYI, since the throttle body boot seals off air before the throttle plate, it would not cause a vacuum leak or high idle.
Does your intake manifold and throttle body look like the one below? If so, I would be highly suspicious of the two vacuum actuators I've pointed to with the red arrows. I'm not very familiar with the pre-88 stuff, but I'm almost certain one of these actuators is for starting air. Here's how it works - there is a spring that holds the throttle cracked open during starting, then once the engine is running, the force of the vacuum on the diaphragm inside the actuator allows it to pull against the spring pressure and close the throttle plate fully. After that, the IACV takes over for controlling idle speed by bleeding air around the throttle plate. That's the intent of how it works, anyhow. The problem is that these cars are so old the diaphragm inside the actuator is almost certainly bad, meaning your throttle plate cannot fully close at idle because no amount of vacuum is going to pull the actuator to close the throttle plate against the spring pressure, and now you have a path for a vacuum leak. I had this issue on a B18A1 engine and was able to remove the actuator, plug the vacuum nipple on the intake manifold and it fixed my issue. You could try temporarily removing the actuator and plugging the vacuum line on your car to see if it works. If you have difficulty starting, you may need to crack the throttle (very slightly) with the gas pedal while cranking.
Also, just FYI, since the throttle body boot seals off air before the throttle plate, it would not cause a vacuum leak or high idle.
Hate to be so late to reply, but I really appreciate your insight! Ended up having some issues with other vehicles I needed to address first but I'm back to tracking down this deal. I'll try plugging that actuator to see if it helps.
Hate to be so late to reply, but I really appreciate your insight! Ended up having some issues with other vehicles I needed to address first but I'm back to tracking down this deal. I'll try plugging that actuator to see if it helps.
Plugging the vacuum line will only potentially solve part of the issue - the vacuum leak at the broken diaphragm. Until you actually remove the actuator from the throttle body, the spring will still hold the throttle plate open, giving you a high idle.
Plugging the vacuum line will only potentially solve part of the issue - the vacuum leak at the broken diaphragm. Until you actually remove the actuator from the throttle body, the spring will still hold the throttle plate open, giving you a high idle.
Ah, I see. I did try plugging the vacuum lines to the diaphragm and didn't notice a difference in idle. HOWEVER, I did make some rather interesting discoveries that I probably should have noticed before. My idle constantly bounces when the car is cold. Once it is at temperature, it shows the behavior I described above. In addition, the heat doesn't blow warm at all but my climate controls do work. I didn't notice until this week, since I haven't driven in the cold. But I did some searching and found that older Preludes will exhibit similar behavior-- bouncing idle and no heat-- when there is air in the cooling system. I'm going to try flushing the coolant and burping the system this weekend to see if that makes a difference. Otherwise, I'm guessing the issue is those diaphragms, and those seem to be rather tough to find for a 1st gen crx. I'll keep this thread updated, but thanks so much for the help so far!
Just to keep this updated, I burped the cooling system. There was a LOT of air in it, so it was definitely a good thing to do. Still, no improvements on the idle though. I traced all of the vacuum lines as well and everything is hooked up properly. So I remain at a loss. My neighbor told me his old RX7 had a similar issue a long time ago, and it turned out to be a "cold start valve" or something like that. Still stuck, but still driving it and loving it. Let me know if anyone has other ideas of what to check!
Idle inconsistency: ideas for diagnosis?
