Hello all! Thanks for taking the time to look at this and thanks in advance for the help.

A little background on the car, it is a 95 Del Sol Si, 5spd that I purchased it back in February and had to put a clutch, axle and a few little odds and ends suspension wise on it. The car is completely stock with the exception of the radio and wheels. I have put probably 500 or so miles on it before running into my current issue.

Once the car is warmed up and the RPMs are in between 800ish-2400ish the RPMs plummet down to 700-1000 and then go back to where they were at. This of course leads to a very hard jerking motion regardless of the gear it is in.

I've cleaned out the IACV, FITV, checked for vacuum leaks, checked the O2 sensor voltage and replaced the throttle positioning sensor.

After replacing and calibrating the TPS, the car ran fine for about 30 miles before the issue came back. I re-checked the voltages and they are within the accepted range and increases smoothly up to WOT.

I'm at the point where it becomes expensive to start switching out parts, so I was hoping someone would have some insight on what to try next. I've read a ton of posts both on here and other sites and I've ruled out (I think ) most of what people have been saying.

I've included 2 videos below that show the issue. When the RPMs are jumping around, I am not letting off of the throttle at all. In the first video I try to hold it at around 1500 RPM and in the 2nd video I am slowly pressing the throttle until it revs normally, never letting off of it until the very end. The 1st video the problem doesn't start happening until the 4.5 minute mark or so as the car was still warming up, but the 5 minute mark definitely shows it and I try to briefly explain the issue. https://www.youtube.com/watch?v=WKvAghs-rx8#t=5m0s


Skip to 5 minute mark

The 2nd video shows it happening through across a larger RPM range.

Thanks again everyone for the time, I greatly appreciate it!

 

CEL codes? Have you bled the cooling system?

When idle is surging:

 

No CEL codes unfortunately. I just tried covering the FITV and IACV holes in the TB and didn't notice any difference. The FITV did have a pretty good amount of suction.

I'm getting ready to try bleeding the coolant system.

EDIT: Bled the coolant system...no bubbles, just a steady stream of coolant. Doesn't appear to have made any difference unfortunately.

I appreciate the help!

 

The engine has a vacuum or intake air leak. Find and fix.

When the engine was fully warmed up or cold?

 

I checked both of them when the car was warmed up and that is when the FITV had a good amount of suction. The IACV didnt have much at all.

The only way to get the surging to happen is to press the throttle and hold it once the car is really warmed up.

I had someone hold the throttle open enough that it started doing the surging and then covered the IACV then the FITV and then both and it didn't make a difference.

I did put my hand over the whole TB opening and the car promptly turned off so I'm guessing it at least isn't an intake leak.

I'll recheck the vacuum lines again and see if I by chance missed one.

Thanks!

 

The FITV hole in the TB should suck no air whatsoever after the engine is fully warmed up. Did you clean and adjust the FITV? Click the FITV link in my signature.

 

I actually went through that post before creating this one. I took the FITV apart, cleaned it (it wasn't that dirty surprisingly) and put it back the way it was since it idles normally when cold/warming up and only really fluctuates when pressing the throttle.

If I take it back off and I adjust it, would I back it out some to reduce the suction? I'm not sure the car will run if it idles much lower.

Thanks again for the help, I really appreciate it!

 

Anyone have any other ideas of things to check or ways to rule other parts out?

 

Here's some more info:
I took the air intake off so I could get to those holes in the TB for the IACV and the FITV. Here is what is weird...when the car is warmed up, but not enough for the fan behind the radiator to turn on, there is strong suction from both the IACV and the FITV. The IACV has slightly more suction than the FITV. This holds true whether idling or giving it gas to the point it does the surging. Now if I cover up both the IACV and FITV during the surging it doesn't make any difference, however once the fan kicks on the surging goes away and the suction for both the IACV and FITV becomes minimal. Then as soon as the fan turns off the suction goes back to strong for both with a slightly stronger suction coming from the IACV hole and then the surging comes back.

 

I had this problem years ago on a 93 LX. My mechanic(ex-Honda Tech) replaced the "thermal wax valve" and that solved the problem. Not sure where it's located, or if that was the actual part name but that's what he called it. Maybe someone else here can shed some light on it.....

 

Sorry for taking so long to reply, I've checked my Fast Idle Thermo Valve and it seems to be behaving properly.

I've discovered a new bit of information. If I unplug the ECT the issue goes away so I assumed that meant the ECT was bad. Replaced it twice in case I got a defective one and the issue was still there. All I have to do is pop the hood and unplug it and the car runs fine.

I know I shouldn't continually run it with that unplugged but does anyone have an idea of what to try next? Could my ECU be going bad? Thanks!

 

Did you bleed the cooling system after replacing the ECT sensor? If so, how?

 

Are your valves out of adjustment? Do you hearing tapping in the top end?

Get a vacuum gauge and hook it up to the manifold.

http://www.cdxetextbook.com/toolsEqu.../vacgauge.html

http://www.motor.com/article.asp?article_ID=457

http://www.international-auto.com/fi...uum-gauges.cfm


Use this to test for

Vacuum leakage

Leaking EGR valve

Leaking head gasket

Also check for clogged air filter, timing belt and/or pulley worn, camshaft lobes worn, fuel pump not delivering sufficient fuel to the injector (buy a B&M gauge or similar)

 

I did bleed the cooling system by leaving the radiator cap off and letting it run for 30ish minutes and the fan kicked on twice during the process.

I bled it again to make sure today and I did notice that blipping the throttle to 2k consistently caused large air bubbles to come through even when none were present while idling.

 

Air filter is new and the issue occurs even with the air intake removed.

Bought a pressure tester and took a couple videos to show what I saw.

In the first on the car was left idling with a few blips here and there. Pressure seems high and from what I've read it indicates a small vacuum leak or ignition issue.
http://youtu.be/XIw3GzEDA8o

Second video was with the car idling at 1400rpm to get the surging to occur. Took quite awhile to get the car warm enough to do it.
http://youtu.be/cG6A7VqNPSg

Posting from mobile...will embed videos when I get to my laptop. Thanks again!

 

A little bit more info. Tried driving with the o2 sensor unplugged to see if it made any difference and it did not. Still surged as bad as ever.

 

Still sounds like a vacuum leak somewhere.

 

I'm confused as to how it can be a vacuum leak if my issue goes away when the ECT is unplugged. I would think that would lend it to being something electrical/sensor related.

I'm starting to think maybe the head gasket has a very slight leak that is letting air into the coolant system and when the air pocket hits the ECT it is throwing everything off. I'm currently trying to figure out how to confirm/rule this out.

I'm sort of grasping at anything at this point so I appreciate the suggestions!

 

I just said it still sounds like one. With weird situations like these it's always something left out, missed, or something else out of left field that winds up being the culprit. But what you're describing doesn't sound like the typical IACV/FITV surging. I've usually only see those happen at idle.

If I were you i'd start checking everything. Even if it doesn't seem related. Stranger things have happened.

 

I didn't mean to come across rude or defensive or anything, I am very grateful for the suggestions. I was more trying to state that from my current understanding I can't come up with a way that a vacuum leak would cause it to behave in that way given the results from unplugging the ECT.

I'm definitely still trying to learn how all of these systems interact so I appreciate any and all explanations on how various things can contribute to an issue like this.

Thanks for the help, I'll definitely try and retrace everything I've done to try and make sure I haven't missed anything.

 

I'm not questioning anything you've done. When I say check, I mean, not just the stuff you did, but possibly the stuff you haven't even touched yet. Switched plugs, wiring, sensors, etc. Like I said, I've seen stranger things be the cause of totally unrelated issues. I've left threads thinking "I don't see how that was possible but as long as ______ fixed the issue."

The other guys may have more tests for you to run but that is my suggestion until they come back.

 

I'm not sure if this will help, but here is what I did to remedy my fluctuating idle on my d16z6.

-I have a block off plate for my FITV, so that's not applicable here.
-remove and clean IACV thoroughly (run brake cleaner through it until the liquid runs clear, let dry, and reassemble)
//This did help restrict the range of fluctuation from 900-1800 rpms down to 1000-1400
Not sure if resetting the ECU helped here or not, but that's what I did to get it to relearn the new idle.

It was still bouncing so next up, I checked out the throttle body. my cable was a bit too tight, and the TB valve wasn't closing all the way. So I readjusted the tension, cleaned the crud off of the spring, and ensured that the valve closed and the surging stopped!
After this, I set the idle down to about 600 rpm and reset the ECU once more. Now, my idle is perfect!

Hope this helps!
-J

 

Hi checking in from 2026 just trying to see if you ever resolved your issue with your 95 Del Sol. I'm currently having this issue and I just ordered a no IACV, but look like I should have done some research first.