Low power when engine hot

On a 2001 super-charged Nissan frontier truck, 6 cylinder, the truck cranks and runs fine until the engine is hot, then seems to lose power. Has no trouble getting up to highway speeds until engine hot, but at that point, it will barely accelerate after it gets hot. Any suggestions on a diagnostics starting point? It's my son's truck and I'm planning on helping him look at it tomorrow. From what he described, I'm thinking of approaching it from a potential fuel delivery, MAF, or restricted exhaust assumption. What is throwing me off, however, is the fact that he says it will get up to normal speeds before it gets hot. He did have some engine coolant temperature sensor codes and he replaced the sensors recently with after market parts. He did say the vehicle is in closed loop when hot. Any experience or advice shared would be appreciated.

You thinking fuel pump problem? The pressure check valve leaking?

Thanks, I will check that. I figured I would monitor the 02 sensor voltages at at wide open throttle to see if they are showing full rich or not to help determine if it is a fuel delivery problem or perhaps a restricted exhaust problem. Talking to my son today, I got one additional piece of information from him. When he first bought this old truck, it had the same problem. One thing he noticed was the truck did not have a thermostat in it when he purchased it. He replaced the engine coolant temperature sensor and that seemed to fix the truck. It ran fine. But then he put a thermostat back in the truck and the same old problem returned. With my limited knowledge, I can't make sense of that in my head. I am thinking maybe there's an intermittent wiring problem somewhere that's getting wiggled at at times.

Oh boy, so no Thermostat when purchased, think you need one with a lower temp.?

Hmmmm, that's a good point. I wonder if the thermostat my son put in the truck had the right temp settings for that vehicle. He's pretty money-conscious (like his dad) so he often shops for the low-priced parts.

Do you have access to scan data?

I would start by establishing if the issue starts when the engine goes into loop. Right off you need to look at the output from the upstream and downstream o2 sensors for both banks and the short and long term fuel trims for both both banks. Also if your scan tool gives them misfire counters

Thank you both. I have the the generic scandata on my auto ingenuity scan tool. I don't have the Nissan subscription to see the mfg specific pids. I spent a little time with the truck this morning and drove it with the the 02 sensors, fuel trim, rpm, tps, maf, and temp sensors recorded on a drive down the street at varying loads. I need to go back and replay the data and look at it more closely, but right off the bat, the 02 sensors were showing full rich at WOT, so I guess that's initially pointing me away from a fuel delivery issue. Fuel trims were looking fairly good for the most part under load, but I noticed a couple of double digit negative trims at idle. I actually didn't see a loop status PID on my scan tool, but my son had previously used his scan tool on a test drive and told me it was going into closed loop while driving down the street. I really appreciate your responses. Paul's videos, book, and premium channel have taught me a lot, but I've got a lot more to learn.. I've got more SD training than I have actual experience at this point.

I could be wrong, but I believe these may have the o2 connectors near each other and the connectors are the same, where they could be mixed up when installed. On the right side of the engine bay.

You are right. When my son first got the truck, he discovered the 02 sensors were plugged in to the wrong connectors. It's nice of Nissan to put the connectors together in an easily accessible location, but it does make it easy to mismatch them when plugging them back in.

timothy.spencer wrote:

I actually didn't see a loop status PID on my scan tool,

Should be "Fuel System n Status". There'll probably be 1 & 2 but 2 might not be used.

I'd also add MAP, and any other pressure PIDS, to the list. Ideally start logging KOEO after a cold soak then drive until the problem appears. Easier said than done with some customers.............

I did some searching and the knock sensor is problematic on these, once a true or false knock is present you loose boost

SD has a video on it, Nissan knock sensor testing

Good suggestions. Thank you. I will try that. I think I noticed on his truck that the knock sensor has been relocated to the top of the engine. Quick question on recording multiple pid's while driving. Is it OK to select all pid's and set the scantool to record while driving? Is or is it best to select only a few at a time and do multiple drives due to refresh rate with many pids selected at once?

Less pids will get your more detailed info than it processing lots of data at once. It depends one what you are monitoring though, since some stuff isn't happening quickly anyway. Sometimes I'll get a bunch of generally related items up, see if anything is grossly out of wack, then narrow down the list.

Do you have the lack of power only under boost?

Thanks. Lack of power only under boost? I'm not exactly sure what that means. My son's supercharger is the fist one I've ever seen.

Try www.clubfrontier.org , I browsed a couple of threads there, great info, one in particular was the supercharger actuator valve test

Oh thanks. I'll check that out.

I mean, is the low power issue only there when the supercharger is supposed to be making pressure?

Oh thanks. To be honest, I really don't know exactly when the supercharger is supposed to do its thing and when it's not supposed to. I did read there was some vacuum transducer on the back of it and I plan to do some vacuum testing on it to see if it seems functional. I also plan to do some restricted exhaust testing, 02 testing, and knock sensor testing.