Suzuki Forenza/Reno hesitation/stumbling symptoms intermittent and no check engine light

My girlfriend has a 2006 Suzuki Reno which is the hatchback version of the much more common Forenza. Mechanically they are the same car. I began to look into this car soon after I began dating her and realized it was simply a rebadged Daewoo product. I attacked the timing belt replacement early on as she had no idea when it was last changed. The old belt was all cracked up and in dire need of replacement when I did the job.

Since then it has just been a lot of small stuff, all things these cars or known for. We are now getting a new symptom. The car sometimes hesitates/stumbles when trying to maintain or gain speed at highway speeds, especially going uphill. There are no check engine lights and I have checked for pending codes. There are none. I figured a misfire would show up but no codes of any kind are present. There was an issue a while back with the evap system purge valve which I cleaned and reinstalled. I don’t know if this could be the issue but it isn’t throwing the P0496 code anymore like it was before I cleaned it.

These symptoms are very intermittent. She used to always put like 3 gallons of gas in the tank when it got near empty and never fill it up. I got frustrated and told her to fill it to the top at least every once in a while. The first time she did this these symptoms started. I wondered if she busted some varnish loose inside the tank that had not been in contact with fuel for some time. We put some fuel injector cleaner in the tank and it cleared up for a while but returned a few tanks later. It seemed to start as she was nearing the end of a tank and continued for a bit after the fill up.

I am getting the feeling this might be a fuel related issue and not the ignition system. I already looked to see if her fuel filter was replaceable and it is not. It is an in tank one that requires the tank be removed. I would do the fuel pump if I had to do this. My other thought is maybe the fuel pump is getting iffy. I am going to do a loaner tool on a fuel pressure gauge if this happens again as I am wondering about that. Also, I haven’t looked into a fuel pressure regulator but that might be something to look into.

Anyone else have thoughts on what might be causing this? I want to get some other ideas before jumping in. This car seems to be one little thing after another if you ask me. It is never anything major but annoying stuff and I guess these cars are known for this.


The conditions it happens is when the engine needs fast delivery of air and fuel, and fast removal of the exhaust products, so the first suspects would be things like the fuel pump, air filter, fuel filter, and clogs in the exhaust system. Spark plug gaps being too wide can cause this too, but the symptom is more often pinging, not loss of power.

It is a pulsing feeling like when an engine misfires. I doubt a plugged cat would give this symptom. It would just be an overall loss of power. It is probably about due for new plugs but why isn’t it throwing a code? I might just do the plugs and see what happens. My gut tells me this is a fuel related problem and you comment suggests much the same. Again, if it was ignition related, wouldn’t there be a code or at least a stored pending code?

Someone else has suggested to me that the O2 sensors could be causing this. This is a good point as I have experienced this problem on at least two vehicles and the mileage is about at the time when a sensor change is in order. Both times I experienced loss of power due to a failing O2 sensor the check engine light didn’t come on. There must be a wide variance these are allowed to perform within before they trip a check engine light. The only time I have ever had an O2 related check engine light on any car was when the wiring to the sensor was severed completely.

Why no code? The rules used in the diagnostic software for posting codes, turning on the CEL vary car to car. Like for misfires, sometimes the rules require a certain number of misfires occur within a certain time period. Otherwise they assume it was a glitch and no code or CEL is posted.