2000 LX with rough idle and bogs on load

[dgz32]

Having some issues with my 2000 LX with 310k miles on it. It’s got a rough idle and the motor bogs when pressing the accelerator. No codes.

This all started when I was initially throwing codes for cylinder 3 misfire, bank 1 too lean, and random multiple misfires. I decided to replace all coils with new Denso units, all new Denso plugs, and a new Hitachi MAF. Also cleaned the throttle body and cleaned the harness connectors to the MAF, TPS, and something else I’m forgetting. Several months ago I also installed an off-brand fuel filter and an AC Delco fuel pump.


Any thoughts? What am I missing? Could it be as simple as a bad fuel filter?

 

[dgz32]

Also replaced the PCV valve, grommet, and both hoses. All other vacuum hoses look ok.

Another thing, I'm getting a MAF reading of 8.5 at idle. Isn't this super-high?

 

[VentricularFib]

Also replaced the PCV valve, grommet, and both hoses. All other vacuum hoses look ok.

Another thing, I'm getting a MAF reading of 8.5 at idle. Isn't this super-high?

Try disconnecting the negative battery terminal for an hour or so to reset ecu so that it learns the new MAF? I had almost the same exact issue, refreshing parts here and there. Bought a MAF sensor from Amazon, some off brand, and had the same thing. Luckily I kept the old oem, reset the ecu, and no more rough idle.

 

[excessive]

I was going to guess MAF , but you already replaced it. It could also be: Bad fuel, vaccum leak, clogged fuel filter or fuel tank sock, many other things. Start with the cheapest / easiest first

 

[dgz32]

Try disconnecting the negative battery terminal for an hour or so to reset ecu so that it learns the new MAF? I had almost the same exact issue, refreshing parts here and there. Bought a MAF sensor from Amazon, some off brand, and had the same thing. Luckily I kept the old oem, reset the ecu, and no more rough idle.

Ah. Hadn't tried that yet. The battery's been disconnected quite a bit lately, but not since I installed the MAF. Will give that a shot tomorrow morning.

 

[dgz32]

I was going to guess MAF , but you already replaced it. It could also be: Bad fuel, vaccum leak, clogged fuel filter or fuel tank sock, many other things. Start with the cheapest / easiest first

I think after the battery disconnect/ECU reset I'll try an OEM fuel filter. New sock on new fuel pump—so I'll wait on that one. But I will say the last fuel pump I put in had a bad piece of fuel line with it. Completely opened up and left me stranded. Was an easy enough fix, though.

 

[medtro]

What is the fuel pressure?

 

[Rebus Knebus]

Also replaced the PCV valve, grommet, and both hoses. All other vacuum hoses look ok.

Another thing, I'm getting a MAF reading of 8.5 at idle. Isn't this super-high?

Yes. Normal is 4.5-5.5.

 

[Rednexus]

These can also have problems with the MAF connector. My GX was running very strange a few months ago (surge at idle, fuel trims oscillating from very high to very low). Removing the MAF connector, cleaning it, and adding some dielectric grease fixed it. It was not throwing any codes while this was ongoing. The connector itself can also be broken inside. A new connector is actually pretty cheap to obtain and easy to splice into the OEM harness - I also had to do that a few years ago when I kept getting MAF-related codes.

The over-reading MAF will make the engine run super-rich and cause most of the issues you are seeing. I'd be curious to see your commanded AF ratios vs. actual, and am betting they are verify different.

 

[dgz32]

Yes. Normal is 4.5-5.5.

View attachment 3399102

These can also have problems with the MAF connector. My GX was running very strange a few months ago (surge at idle, fuel trims oscillating from very high to very low). Removing the MAF connector, cleaning it, and adding some dielectric grease fixed it. It was not throwing any codes while this was ongoing. The connector itself can also be broken inside. A new connector is actually pretty cheap to obtain and easy to splice into the OEM harness - I also had to do that a few years ago when I kept getting MAF-related codes.

The over-reading MAF will make the engine run super-rich and cause most of the issues you are seeing. I'd be curious to see your commanded AF ratios vs. actual, and am betting they are verify different.

Interesting re the connector. I did clean it with some electrical spray...but will try the dielectric grease. Was getting the same readings from my original MAF, so that may very well be the culprit.

 

[Rednexus]

Interesting re the connector. I did clean it with some electrical spray...but will try the dielectric grease. Was getting the same readings from my original MAF, so that may very well be the culprit.

I dug thru my emails. I had purchased a new connector from Ballenger Motorsports. It was $15 and is shared with multiple other types of non-Toyota vehicles. Definitely replace it if there is a question about it.

 

[dgz32]

I dug thru my emails. I had purchased a new connector from Ballenger Motorsports. It was $15 and is shared with multiple other types of non-Toyota vehicles. Definitely replace it if there is a question about it.

Disconnected the battery for a couple hours and used dielectric grease on the MAF plug. No joy. Pins on the plug looked to be in good shape and no visible breaks in the wiring. Leaves me with bad MAF plug (since it's still reading double over what it should be reading). Or maybe injector, or fuel pressure problem (need to look at that).

 

[dgz32]

I dug thru my emails. I had purchased a new connector from Ballenger Motorsports. It was $15 and is shared with multiple other types of non-Toyota vehicles. Definitely replace it if there is a question about it.

Thanks, I'll order one today.

 

[dgz32]

What is the fuel pressure?

I have no idea—I'll try to check it today.

 

[Rednexus]

It could also be a vacuum leak, if the plug does not fix it. Those can be found by spraying carb cleaner on all vacuum connections and watching for negative spikes in STFTs.

However a vacuum leak will usually throw a code.

 

[dgz32]

Well—the new MAF plug wasn't the fix I was hoping for. It drives fine for the first few seconds then will start to stumble like a cylinder miss—and pressing the accelerator results in the vehicle barely moving. If I turn it off and immediately back on, it drives fine for the first few seconds, then back to the stumble and no acceleration. When I turn it off I get a faint fuel smell. I'm wondering now if it could be the TPS. Also, what are the chances of a bad MAF AND a bad replacement MAF? I'd imagine pretty slim...

So far I've replaced the following: MAF, MAF connector, all plugs, all coils, fuel pump, fuel filter, PCV valve w/grommet and both hoses. Cleaned throttle body and reset the ECU. Pretty soon I'm gonna have a new vehicle.

Borrowing a neighbors fuel pressure gauge tonight to check FP so I can rule that out.

I need take a good look at everything again tonight. Was reading where a user found a loose injector connector which was causing a similar problem.

 

[dgz32]

Looks like I need to add a fitting in order to actually check the fuel pressure—so I've ordered parts to do a flintknapper-style gauge install—which'll have to wait until the weekend.

The LX ran great, though. Was able to drive it a good bit with absolutely no stumbling. I rechecked after the drive and found a pending P0171—bank 1 lean code.

​

 

[dgz32]

I reset the previous P0171 code and drove around with no issues for a good half hour. The code re-appeared about 35 minutes into the drive but drove fine for the most part with a dead spot in third gear. Arrive at my destination and shut the vehicle off. When I cranked it again, I got the stumbling idle. Turned it off and back on—it drove fine but with that dead spot like before. Light still on.

So I get home and drive it around with a scanner hooked up—I noticed that Bank 1, Sensor 2 wasn't reporting anything at all for Short Term Fuel Trim. And the overall Short Term % Trim for Bank 1 is a full 5+% lower than bank 2 (14.06% vs. 19.53%). Could the bank 1 downstream O2 sensor be at fault? I'm wondering if it's sporadically reporting STFT and when it goes offline—that's what trips the code. Both upstream O2 sensors are new. Thoughts?

 

[Malleus]

Use the FSM troubleshooting procedure; way cheaper than the parts cannon.