netfreak

Got an issue where between the 2500-3500rpm range (possibly elsewhere but not as noticeable) when I stop applying throttle I start to feel some sort of surging, kind of like if I were to tap the throttle over and over. Looking for possible causes. Recently checked the TPS and the voltages seemed to check out, but I do have a new one I can try installing. Could this also be a ignition problem? Actual acceleration seems to be normal and this problem only happens when no throttle is applied.

EDIT: Some other possibly relevant data... There is no CEL, car idles perfectly, revs fine, doesn't SEEM to be any dead spots in acceleration. O2 sensor is fairly new, IACV was recently fully cleaned out, fuel filter maybe about 3 years old, fuel pump is new (the issue started before I upgraded my fuel pump this week). Spark plugs looked fine last time I checked and I popped the distributor cap off but nothing seems obviously wrong in there. I'll likely replace the TPS anyway since I have a new one and my current one came from a junkyard.

2x0

What fuel injectors are you using? Sounds like you are hovering around the point where the injectors shut off under decel. I've had this happen at low speeds in 1st or 2nd gear when trying to maintain speed, injectors shut off and causes a sort of "bucking" effect.

netfreak

I'm using the stock GSR injectors. I don't have any spares to try at the moment though. The issue is less with maintaining speed though and more with when I fully release the throttle.

2x0

If TPS and throttle are good then I would suspect a vacuum leak. Something is letting it get enough air that the injectors aren't staying cut like they should.

netfreak

I'll try checking those but every vacuum line and hose under the hood is less than 2 years old - I'd be surprised if those were already failing.

Replacing my IACV with a clean one is the one thing I did shortly before this issue started happening. While it did fix an existing idle issue, perhaps this is related?
EDIT: in terms of the gasket... I think I used an old one.

netfreak

OK seems that the problem is getting worse and it feels like I've got misfire under partial throttle. Going to start swapping fuel/ignition parts and see what happens I guess.

netfreak

Yeah it seems the ICM was the root cause of the issue. I'll have to do more testing but threw a junkyard one in and I didn't encounter any problems after a couple hours.

netfreak

OK so I was wrong and it was in fact my "new" IACV somehow. Put the old one back in and while my idle is sometimes a little weird, the issue has not come back after doing a few hours of stress testing. Posting this to help anyone else with this problem who may stumble across the thread.

netfreak

Oh god so I'm wrong again. Turns out the problem actually only starts happening when the ambient temperature is below around 20c, which explains why my daytime road tests are fine but the problem creeps up at night. I'm about to go replace any sensor that has anything to do with air/temperature but wondering if anyone else has seen anything like this...

holmesnmanny

for the record, what was the solution ?

netfreak

Still working on it unfortunately. I thought I had it solved earlier but then discovered the problem only starts happening when the outside air is cooler. I'll have time next week to swap more parts and test.

holmesnmanny

you might wanna edit "solved" back out

Caoboy

IAT?

netfreak

I'll be trying a spare map and IAT tomorrow and test driving in the cold. I'm not able to update the title of the thread so I'll just keep going until it is actually solved

EDIT: replaced the map and IAT. The problem did come back for a couple minutes but I drove another 3hrs in the middle of the night and it stayed away..... so potentially this is now fixed but I'll have to see how it is over the next week or two.

netfreak

Nope, problem started immediately when I took the car out this morning and was in traffic. When the road opened I did a WOT pull and the problem went away for a few hours though... As it came back later in the day the same thing made it go away for a while. I'm going to swap the fuel injectors next.

holmesnmanny

exactly what kind of chassis, engine, harness, any other relevant info are we talking about here ?

netfreak

91 CRX Si, B18C1 bottom end, B16A top, OBD1 P28 with basemap for this motor. The swap is almost 2 years old now I think and this problem started happening maybe about a month after I started driving it after last winter (sits there for a few months). Pretty much everything under the hood is new such as vacuum lines, O2 sensor, drivetrain components. I've swapped the following with apparently no impact on the issue: ICM, cap/rotor, TPS, MAP, IAT, IACV, fuel filter, new fuel pump (Walbro 190). I thought there was a pattern to when it would start happening but now it seems more random but partially based on my throttle usage. It appears to happen when I'm stuck driving under light throttle such as in traffic or just cruising down the highway. My working theory at this point is a defective injector somehow allowing fuel in when it shouldn't be. Basically it feels like someone is tapping the throttle over and over while I'm rolling down the road, but I'm also feeling that the powerband is inconsistent at times and it SEEMS like I might be missing a cylinder... or something. My latest finding is running through a couple gears at WOT makes the problem go away for a while. I've got spare injectors I'll be trying next week anyway.

2x0

netfreak

I'll look into that. When I installed the new one I calibrated it at the sensor plug end.

holmesnmanny

was this car ever working perfectly after the swap before this issue started ? if you are still on a basemap then that could be the issue, you need a real tune

2x0

netfreak

The car ran fine with the swap for about 1.5 years and the only notable thing prior to the issue happening was it sitting over the winter (I'd occasionally start the car but was not driven). I have a basemap chip for the ECU as close to my engine specs as they could and has not caused me any issues.

2x0

netfreak

It also doesn't happen all the time. Spent the other day on the highway for like 8 hours and I could feel the problem happening maybe 5-10% of the time tops. I have spares of probably everything under the hood though so I'm bound to track it down.

holmesnmanny

It's not a stock engine if it's got a different head in it.