Exhaust & engine temp

[Matt 4.3 TBI]

Just been brainstorming some more, I"ll see if I can word this properly so it don't sound crazy!

I've been noticing lately that my temp gauge will go up one bar (10 degrees) when I'm cruising over 70-75 on the highway (about 2000-2250 rpm). I've been fighting with an error-prone gauge, but that only occurs when the stereo is on. I still haven't figured out WHY it happens, but I do know that if I turn off the stereo it will drop 10 degrees.

This isn't that problem, as it will go up when it's over that speed with the stereo off. I've been racking my brain as to why it would be overheating, and I came to the exhaust. I remembered I still have a 2.25 tailpipe from the muffler exit back. Do you think this restriction and backpressure would be enough to cause a 10 degree shift? The temp will drop back to normal (about 180) when I keep the speed under 70 or 2000 rpm.

I've been thinking about having an electric cutout welded in between the cat and muffler, but as far as I know, they are illegal in TX, even if they are capped on the road. That would be the best of all worlds for exhaust- a full system for everyday driving, then flip the switch and open it up for a run at the strip, or a few more horses and mpg on the highway.

[Matt 4.3 TBI]

Another note-
I only expect the exhaust isn't flowing enough due to all the work done to increase airflow through the engine (gasket matched ports, intake port matched 2" into the manifold, 3-angle valve jobs, larger lift and duration cam, plus the open-element and headers)

[Sparky2263]

Hey Matt, the temp. sender is in the cylinder head. The coolant sensor is right before the outlet to the radiator. The sensor would be the more accurate of the 2 anyway. Why not measure the sensor voltage (or view it on a scanner) to see if it correlates to the sender? I'd bet it's real steady (within .2 volts) once warmed up.

Our gauges (as you know) are notoriously inaccurate. But, they're good for trends.

My voltmeter reads 9vdc with key on and a little over 11 running. Alt. output is 14.3

[Matt 4.3 TBI]

I watch it with WinALDL, as it reads the voltage from the CTS, not the sender. The sensor shows no increase from the mysterious stereo interference (though the gauge will sometimes show up to 20* over normal), and CTS shows a steady 179*F. I haven't checked this late development against the CTS, as I need a good stretch of highway to reproduce the problem. And oddly enough, today I had to make a trip 55 mi each way, avg about 75, and the gauge didn't fluctuate at all. I really hate our gauges! I've already given up on the fuel gauge, but I really don't like not knowing the coolant temp.

[blazen]

your only talkin it is going up 10 degrees when your going over 75. 10 degrees isnt bad it could just be because your workin the motor alot harder to go 75 than you would to go 65. especially since you have a first gen there not very airodynamic. plus it will also go up in temp if you tail gate alot because when your alot closer to the person in front of you your not getting as much air flowing through the radiator. and a clogged exhaust will proubly make it run hotter i dont think its gonna get hotter just because you have a 2.25 tail pipe.

[Matt 4.3 TBI]

Well, here's the deal, I had a full 2.5 exhaust for a while before I rebuilt the engine, and it never saw over 180 (had it up to 128 before, temp still stayed solid). Also, right before I did the exhaust I was burning plugs. After I put on the high-flow cat, flow40, and 2.5 pipes it solved that proplem.
About a year later I got lured into the darkside theory of backpressure when looking for more low-end grunt, and had a muffler shop bend me a new tailpipe. Before the build (on the 112k mile motor) it seemed to help the low-end by shifting down the torque curve a bit, and I picked up about 1mpg. Almost every weekend I drove between Ft Hood and Dallas (about 3 hrs, 200 mi) at 80-90 and I never saw my gauge rise.
Ever since the build by highway miles have been down, sometimes worse than city. I try to keep it under 2k most of the time, so I can't really say it's "working the motor a lot harder". It does 75-80 at about 2250. Around the same speed I cruise at doing 20 in 2nd. Now when I drop a gear or two to get it over 100, then it starts screaming and I'd expect the temp to bump up a bit. Maybe I'm just chasing a ghost here, so I'm just throwing out ideas.

[Sparky2263]

Here's an idea (aside from hooking a DVOM to the sender and watching it), temp. sender's are cheap, right? Screw a new one into the rh head and ck. 'em left to right. I'd be curious to know the results of that one.

[Matt 4.3 TBI]

Heh, I've got a new one right now that I've been meaning to install. Had to go get a new wrench as I can't get at the sender with any sockets now that the headers are there. The guys at the engine shop must've use some locktite instead of teflon tape, tho. Thing's trying to take root.

[Sparky2263]

I was thinking have 2 senders. One in each head and ck. 'em side to side.

[Matt 4.3 TBI]

Yeah, I know what you mean, not sure if I could get that plug out, though. Hell, I'm not even sure if I can get a DMM to the one that's installed since the primary goes right in front of it. On the driver side I just have to fight with the steering shaft. There's no way the gauge wire would reach the driver side head, either. That'd be a nice fix, though.