Car stumbles/surges when in gear?

1994 TOYOTA COROLLA

326,000 MILES • 1.8L • AUTOMATIC

FORENSICS

MEMBER
112 POSTS

So last month my Corolla broke down. On my way to work getting off of the freeway my car started losing power and would barely accelerate and it worsened the more I pressed the pedal. I got it home and started testing for spark, timing, etc. I found that priming the fuel pump several times and then starting it would allow it to run for 4 or 5 minutes and I could still hear the fuel pump so I replaced the fuel filter and boom runs great for 3 days and now after doing anything above 40mph at idle it will randomly stumble and worsens if it’s idling in gear. I ordered a brand new Denso fuel pump and regulator just to see. I don’t have any acceleration issues or power loss or dying while driving but I’m kind of stumped on what it is without throwing the parts cannon at it. Oh and if it helps, before replacing the fuel filter any time I could get it to start it smelled real funny I don’t know how to describe it but it didn’t smell right.

Aug 11, 2026 at 1:32 PM

FORENSICS

MEMBER
112 POSTS

Here are two videos of the symptom. You kind of have to listen closely

Videos (Click to enlarge)

Aug 11, 2026 at 2:14 PM

JACOBANDNICKOLAS

✓Experienced Mechanic
110,240 POSTS

Hi:

If the fuel pump was replaced and the pressure is within specification, my next thought would be to check for a restriction in the exhaust system, specifically the catalytic converter.

The catalytic converter can produce a rotten egg smell if it is failing. Take a look at this link and check this to see if it appears functional.

https://2carpros.com/articles/how-to-test-a-catalytic-converter/

Additionally, if the fuel regulator is no good, it may be getting too much fuel, which can also cause a strong odor from the exhaust.

Let me know.

Take care,

Joe

Aug 11, 2026 at 7:33 PM

FORENSICS

MEMBER
112 POSTS

Hello, I got the new regulator in the mail yesterday and that led to no change, the new fuel pump arrives tomorrow so I’ll be able to do that soon. Today I got a new ignition coil which I’m hoping is the culprit, and a new IAC because autozone had one and I can check it off. Here’s a picture of the coil though as you can see there is a crack in the gel and it appears to have maybe melted a bit by the metal contact. Also it’s original. Thanks a bunch Jacob, what do you think about the coil. Ever seen one stop working then start mostly working again?

Image (Click to enlarge)

Aug 13, 2026 at 6:07 PM

STRAILER

✓Experienced Mechanic
55,214 POSTS

First of all I need to say I can appreciate the way you take care of your car, the engine is clean as a whistle, I like that, to me it sounds like the catalytic converter is coming apart and clogging the exhaust outlet for time to time. Pleas use a rubber mallet and tap on the exeats to see if you hear loose particles inside the system. This would account for the smell issue as well. Please check out this guide to see what I am talking about.

https://2carpros.com/articles/bad-catalytic-converter-symptoms/

Please let me know.

Aug 15, 2026 at 6:46 AM

FORENSICS

MEMBER
112 POSTS

Hey so after a good week I did discover that it was in fact the ignition coil. I took my multimeter and did the whole testing just to confirm. However replacing the IAC, cleaning the EGR again (I’ve cleaned it every 3ish years as it burns a whole lot of oil) and doing the rest has left the car running much better than it did. Since replacing the coil I have had no issues so hopefully it stays that way.

Aug 18, 2026 at 3:35 PM

STRAILER

✓Experienced Mechanic
55,214 POSTS

Hmm, yep that was my bad, I can see the ignition coil is bad. Glad you got it fixed.

Aug 19, 2026 at 6:28 AM

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