Hey guys, have a bit of an issue I need to resolve hopefully by tomorrow night. Right now my car idles perfectly 800-900rpm. However, when i give it a bit of gas it starts to surge between 1000rpm and 2000 rpm. If i give it gas to close to 3000 rpm it slows down back to a perfect idle. If i drive the car it accelerates fine but upon decelerate it jerks the car forward.

What could be causing this? The IACV was thoroughly cleaned, a new filter and a new gasket. The fast idle valve was cleaned as well. It has a rosko TB and IM gasket. I'm thinking it could be a small vac leak, or possibly has air in the coolant system still. I need to bleed the system again, but I am looking for all suggestions on what it could be so I can get this fixed tomorrow. I have to drive it for an exhaust and alignment on Friday morning and would prefer the car running properly before I drive to a shop open header.

 

https://honda-tech.com/forums/honda-prelude-4/end-all-idle-surge-bounce-etc-problems-thread-1979500/

Forgot about this thread, this should be a good start.

 

If you're driving along at low RPMs [<2000] and it intermittently seems to drop out, you probably have a problem with your IACV.

You can unplug the IACV and drive around to feel what I am talking about. You'll get a CEL and when you slow down the motor will feel like it completely drops out.

What is happening is that with the throttle plate closed there is no way for the computer to let a little extra air in to even out the off throttle running of the engine.

There are two possible solutions to this problem.

1) There is air in your coolant
2) The IACV is bad

To make there is no air in your coolant
Jack the front of your car up to where the radiator cap is higher than the bleeder
Place jack stands appropriately
Open the bleeder, which on the thermostat housing on the top left at the back of the motor
Fill your coolant til it runs out of the bleeder
Close the bleeder
With the radiator cap off, still jacked up, run the car for 5-10 minutes, adding coolant as necessary
Shut the car off, fill the radiator and top off the overflow bottle
You should be done if the IACV is good


You can help this process by squeezing the lower hose with your hand while its running to help push air bubbles into the radiator. You will know you don't have any more bubbles in the lower hose because it will become hard to squeeze.
You can also squeeze the other hoses that go to the FITV and the IACV to see if they are getting filled.

Also, your radiator fan should come on during this process, if it doesn't it could indicate a bad thermostat.

You will spill coolant, be very careful, the temperature of coolant easily exceeds the boiling temperature of water.

Otherwise you have a bad IACV.

I have a few of them lying around.

 

Awesome, thanks for the tips. I'm just about to go and tackle this task right now.

 

Good luck!

 

Update;

Tested everything. It is for sure a vac leak some where on the intake manifold and cylinder head, tb, or butter fly plates (deleted them).

TPS works, IACV works, FITV works, Coolant was bled until zero air was in the system leaving only the vac leak to be the culprit. Tomorrow I will remove the intake manifold, open everything up again and torque these bastards even more. Not a big deal to me since I need to fix an oil leak from a sandwich plate. I can now remove the intake manifold in about 30 minutes in a personal garage lol. I've removed it one to many times.

 

Haha, been there with the IM. Discovered I can get away with leaving a couple of the lower IM nuts off to ease install and removal without causing a vac leak. Careful of overtorquing bolts or studs; breaking them will only amplify your problem since not only will you not be able to torque them down at all (possibly causing a vac leak again), but then you have to extract them. Putting in new gaskets (again) is not a bad idea since its cheap and eliminates one potential problem. I usually use some grey RTV on both sides also and smear it around on all the outside edges.

 

Before you take it all apart, try letting the engine run and spraying carb cleaner on your vacuum mating points.

When the idle raises you've found your vacuum leak.

 

We actually did that but could not find any from above. It has to be from below.

I have one of those Rosko Racing thermo gaskets (intake mani and throttle body) and they are reusable. However, i've torqued it down a few times and it left an imprint in the gasket. I think it may be causing a leak. I have an oem honda one as well, but I think I am going to throw on a bunch of gasket maker along with the Rosko gasket. I'm hoping this takes care of the issue once and for all. I'm sick of working on the car, i just want to drive it haha.