Heat shields and misfire (1 Viewer)

[musthave]

Began working on a customers Landcruiser last week. He had an intermittent 300 code, 305 code, and a slight shuttering and lack of power when the vehicle was hot. Most notably when first accelerating after a stop, and then again around 2000 RPM.

With no previous service records, I went ahead and replaced plugs, wires, distributor cap, rotor, and coil. I also replaced the air doctor because it was cracking and brittle.

The shuttering affect continued. And if driven enough miles, the 305 code might appear. It did a total of two times. That is with over 300 miles in test drives trying to duplicate the issue.

The issue only occurred when the vehicle temperature was 186 or higher, the air intake was over 120.

I hadn’t paid attention to the fact that his heat shields were gone. I believe that this caused the temperature of the mass airflow sensor to increase significantly above normal, and caused the issue.

After installing new heat shields, I have not been able to duplicate the issue. To test my hypothesis, I took off the heat shield and brought the temperatures back to where it was experienced originally. Sure enough, it through a 305 code, and the shuttering persisted.

I reinstalled the heat shields and have driven several miles without any further issues.

The customer has had their vehicle back for almost 2 weeks now, no further issues.

I’m not 100% convinced that I have isolated the issue but it appears that I have. Has anyone else come across something similar?

 

[BILT4ME]

300 / 305 Codes are a cylinder misfire (305 is specific to cylinder #5)

A Mass Air Flow Sensor could be the root of it, however, that feels like grasping at straws to me.

What was the wiring harness like by the EGR? It could be the higher under hood temps without the heat shields cause a wiring harness to droop a bit more and short out, making it hard to find.

That's a tough one, but if you let the customer know you didn't find anything specific, it may or may not be resolved.

What about spark plug tube seals? Any oil in and around the plugs?

A cylinder misfire could also be an early indicator of HG.

 

[musthave]

300 / 305 Codes are a cylinder misfire (305 is specific to cylinder #5)

A Mass Air Flow Sensor could be the root of it, however, that feels like grasping at straws to me.

What was the wiring harness like by the EGR? It could be the higher under hood temps without the heat shields cause a wiring harness to droop a bit more and short out, making it hard to find.

That's a tough one, but if you let the customer know you didn't find anything specific, it may or may not be resolved.

What about spark plug tube seals? Any oil in and around the plugs?

A cylinder misfire could also be an early indicator of HG.

Absolutely, 300 random misfire, 305 #5. I don't recall the temperatures when I scanned the bottom of the MAF before and after, but it was a 30 degree difference.

The HG was done 20K or so miles ago (I wish they would have removed EGR but it was from CA so I get it). The harness had been well wrapped by someone previously and I didn't look further at it. (Though there are a lot of threads that speak of the 305 code and the wires in the harness next to the EGR). I did put a zip tie or two to try to hold the harness a bit better to prevent that drooping. I also added some heat shield tape loosely around the harness over about 6 inches. Compression is excellent across all 6 all within 1 or 2 of each other. No signs of HG issues.

I let the customer know exactly what was done and that if it comes back we'll look further. I initially told them that I suspected HG right from the start, but after knowing that it was done recently I started looking elsewhere.

Plug 2 had more residue than the others but all of them were dry. #2 could have been wet from a puddle in the past. The wires that were on it were some off brand ones.

I guess what intrigues me is that after I took off the shields I got the 305 again and I got the shuddering/stumble. When I put them on the 2nd time, it never happened again. Coincidence? I don't know. The ONLY time I ever got the 305 and the shuddering was when the vehicle had been ran for 20+ miles, had an engine temp above 186 (never over 192), and only when the air intake was over 120. Which tells me that the engine bay is hotter than what I would consider normal (around here).

The heat shields play an important role, probably more than I realized.

Customer completed a drive to Kentucky and arrived there yesterday. 900+ mile ride. No code, no issues.

Hopefully it doesn't return, if it does, then my hypothesis is shot.

 

[BILT4ME]

Absolutely, 300 random misfire, 305 #5. I don't recall the temperatures when I scanned the bottom of the MAF before and after, but it was a 30 degree difference.

The HG was done 20K or so miles ago (I wish they would have removed EGR but it was from CA so I get it). The harness had been well wrapped by someone previously and I didn't look further at it. (Though there are a lot of threads that speak of the 305 code and the wires in the harness next to the EGR). I did put a zip tie or two to try to hold the harness a bit better to prevent that drooping. I also added some heat shield tape loosely around the harness over about 6 inches. Compression is excellent across all 6 all within 1 or 2 of each other. No signs of HG issues.

I let the customer know exactly what was done and that if it comes back we'll look further. I initially told them that I suspected HG right from the start, but after knowing that it was done recently I started looking elsewhere.

Plug 2 had more residue than the others but all of them were dry. #2 could have been wet from a puddle in the past. The wires that were on it were some off brand ones.

I guess what intrigues me is that after I took off the shields I got the 305 again and I got the shuddering/stumble. When I put them on the 2nd time, it never happened again. Coincidence? I don't know. The ONLY time I ever got the 305 and the shuddering was when the vehicle had been ran for 20+ miles, had an engine temp above 186 (never over 192), and only when the air intake was over 120. Which tells me that the engine bay is hotter than what I would consider normal (around here).

The heat shields play an important role, probably more than I realized.

Customer completed a drive to Kentucky and arrived there yesterday. 900+ mile ride. No code, no issues.

Hopefully it doesn't return, if it does, then my hypothesis is shot.

When you R/R the heat shields each time, did you remove the MAF wiring harness or plug? Or just work underneath the installed intake hose?
Maybe there's something in that harness or the movement of the MAF that could be affected by the joggling of the parts.

Good luck! I hope it stays away!

Do you know if the HG was a Toyota HG or aftermarket? Some of the aftermarket HG are known to fail, but if your compression numbers are good and that close, maybe not.

 

[musthave]

300 / 305 Codes are a cylinder misfire (305 is specific to cylinder #5)

A Mass Air Flow Sensor could be the root of it, however, that feels like grasping at straws to me.

What was the wiring harness like by the EGR? It could be the higher under hood temps without the heat shields cause a wiring harness to droop a bit more and short out, making it hard to find.

That's a tough one, but if you let the customer know you didn't find anything specific, it may or may not be resolved.

What about spark plug tube seals? Any oil in and around the plugs?

A cylinder misfire could also be an early indicator of HG.

View attachment 2692557

When you R/R the heat shields each time, did you remove the MAF wiring harness or plug? Or just work underneath the installed intake hose?
Maybe there's something in that harness or the movement of the MAF that could be affected by the joggling of the parts.

Good luck! I hope it stays away!

Do you know if the HG was a Toyota HG or aftermarket? Some of the aftermarket HG are known to fail, but if your compression numbers are good and that close, maybe not.

When I R/R the heat shields I did not remove the MAF wiring harness or plug. I just worked underneath the intake hose.

The receipts he had for the HG stated only that the HG was replaced along with some other parts in December of 2019. They charged him $4300 which seems a bit high to me. Perhaps that's fair out in Cali though. It didn't mention OEM or not on any of the items. That said, the fan clutch was OEM and I believe the belts were OEM.