hot and rough, running that is

GTManley · Apr 1, 2026

Hey folks GT here with a delima that has me going crazy.
Truck (1988 1500 4wd suburban, stock tbi 350) runs hot after driving and now has started to idle rough as all get out when under load but returns to a good idle if put in neutral or park.
Here is the back story and what ive done so far.
I noticed one day about a week ago that after a 15 minute drive home i reach the operating temp (usually around 210) but after heading back out to drop my kid off at practice the temp climbs to right under 260, not a fast spike like low water but a steady climb over another 15-20 minutes or so, i reach my destination before it boils over. first thought was cooling system but its all been replaced over the last year ( radiator, water pump, fan clutch, upper and lower hoses, heater core, radiator cap, thermostat twice - first to a 195 stat and then back to a 180 stat and not water level stays where it needs and not losing) and if i let it sit idle in park but still running the temp drops back to normal after about 10 minutes. Next move was to pull a plug and noticed its running extremely lean according to what the plug looked like so here is where i really dove into it.
first off i checked timing and it was spot on (newish cap, rotor, module and coil within the last 2 years), then decided maybe a clogged or weak fuel system so i replaced the fuel filter and no change, then i dropped the tank and replaced the pump assembly still no change. this whole time the truck has operated smooth not jerky or rough idle and no stutter on takeoff. Last night i changed the EGR and checked all the vacuum lines, replaced the line from the solenoid to the egr because it snapped during assembly. i did drop a can of sea foam in the tank then went for a test drive, was able to drive a little more time before the temp started to climb but now at idle in gear it struggles to stay running still drives good just not at a stop. i assume its running rough right now because of the seafoam but i still cant figure out why im still running hot after driving and im running out of where to look for a solution. right now im about 2-3 months out from a new engine replace, just trying to get by without putting all my new parts on a worn out block but still need to maintain a daily driver status till the swap happens.

thank you in advance for any insights or solutions
GT

YakkoWarner · Apr 1, 2026

The lean running is probably the cause of the heat buildup. I'd check fuel pressure but thats not always a good indicator because it might be fine while idling in the driveway, but starting to starve out while driving (and theres no way to drive it with the hood up and test gauge attached). Could be a lazy injector or the pressure regulator inside the thottle body getting weak.

goldpack · Apr 4, 2026

what does it have for a thermostat, what does it have for a rad cap.

what does it have for radiator hoses.

all those things get old and don't work like they should. (thermostat and rad cap are spring steel, and may not open and close properly,
at the right temp or flow / vent enough...aftermarkets may just suck and be out of spec)

as for the hoses. I wonder if one of them is getting vacuum sucked closed from heat / flow

GTManley · Apr 6, 2026

upper and lower hoses are both new and do not appear to suck shut during operation, as for the thermostat i have tried a 195 and a 180 with no change.
ive cleaned the Iac and tps to the best of my ability, next step is to throw a new dizzy on it and see if that alters anything. i did find a couple of vacuum leaks that helped some but still getting a erratic idle sometimes its correct but then it either runs super high or drowns out to nothing and causes the engine to sputter

goldpack · Apr 7, 2026

the other day I was reading on a car forum it was possible to get a throttle positon relearn on a tbi 305,350 1988 era car.

early stuff but had some directions ...

Wonder if all is gummy from varnish,... or just plain old springs

Just spitballing ...and bumping the thread.

You might try charm.li for a fsm ... maybe find a troubleshoot checklist, ...those are typically cited. From Liechtenstein...

YakkoWarner · Apr 8, 2026

I had to do that on my 1989 truck when I replaced the throttle body - its not hard but is fantastically easier if you have a cable/laptop setup to read the data from the computer directly. The basics are:

Run the engine so its at operating temperature.

Jumper the pins on the ALDL connector as if you're reading the codes, this will also make the idle control close completely. While the codes are flashing, disconnect the idle control motor so that it stays 100% closed (when you turn the key off it automatically repoens, hence the need to disconnect while codes are flashing). Once disconnected you can turn off the key and remove the jumper. Leave the idle control disconnected.

The next parts are where having the laptop setup is really useful, because you can see in near-realtime what the computer is registering for coolant temp, idle speed and TPS voltage.

Restart the engine (you may have to feather the throttle to keep it running) and adjust the base idle screw at the lower passenger side front of the throttle body to obtain an idle about 150-200 below the computer's programmed idle (400-500RPM is a good baseline). The computer won't be able to raise the idle because the idle control was left disconnected.

Once the base idle is set, turn off the engine. Now adjust the TPS to have as close to exactly 0.5V with the throttle fully closed (use the adjustment screws if its an adjustable type or gently and very lightly bend the tab that goes into the throttle mechanism if not adjustable). Open and close the throttle a couple times and make sure it comes back to 0.5V or very close (try to get between 0.45 and 0.55 at least).

With the closed throttle reading 0.5v on the TPS, it should change to somewhere between 4.8 and 5V when the throttle is fully opened.

Reconnect the idle control on the throttle body. Then restart the engine, and the computer should raise the idle to around 750-800 depending on the programming. At this point the TPS should be calibrated and unless something slips or fails it should stay in spec.

JBswth · Apr 27, 2026

GTManley said:
Hey folks GT here with a delima that has me going crazy.
Truck (1988 1500 4wd suburban, stock tbi 350) runs hot after driving and now has started to idle rough as all get out when under load but returns to a good idle if put in neutral or park.
Here is the back story and what ive done so far.
I noticed one day about a week ago that after a 15 minute drive home i reach the operating temp (usually around 210) but after heading back out to drop my kid off at practice the temp climbs to right under 260, not a fast spike like low water but a steady climb over another 15-20 minutes or so, i reach my destination before it boils over. first thought was cooling system but its all been replaced over the last year ( radiator, water pump, fan clutch, upper and lower hoses, heater core, radiator cap, thermostat twice - first to a 195 stat and then back to a 180 stat and not water level stays where it needs and not losing) and if i let it sit idle in park but still running the temp drops back to normal after about 10 minutes. Next move was to pull a plug and noticed its running extremely lean according to what the plug looked like so here is where i really dove into it.
first off i checked timing and it was spot on (newish cap, rotor, module and coil within the last 2 years), then decided maybe a clogged or weak fuel system so i replaced the fuel filter and no change, then i dropped the tank and replaced the pump assembly still no change. this whole time the truck has operated smooth not jerky or rough idle and no stutter on takeoff. Last night i changed the EGR and checked all the vacuum lines, replaced the line from the solenoid to the egr because it snapped during assembly. i did drop a can of sea foam in the tank then went for a test drive, was able to drive a little more time before the temp started to climb but now at idle in gear it struggles to stay running still drives good just not at a stop. i assume its running rough right now because of the seafoam but i still cant figure out why im still running hot after driving and im running out of where to look for a solution. right now im about 2-3 months out from a new engine replace, just trying to get by without putting all my new parts on a worn out block but still need to maintain a daily driver status till the swap happens.

thank you in advance for any insights or solutions
GT

Just a hunch, but what kind of EGR valve do you have? The conventional vacuum operated kind or the exhaust back pressure modulated kind? Does your EGR vacuum line come off of a switch that has 4 vacuum hoses coming off of it? My 81 C10 6 cyl had such a set up, and seemingly two of the hoses were, in the emissions sticker, reversed. With the original EGR valve, no problem, but after I replaced it, it idled very rough and lean, but with the vacuum line removed from the EGR valve, it smoothed out, (it was receiving vacuum even at idle), so I swapped those two that I thought were mis-printed, and that fixed the problem, and the smog check guys never had a problem with it. Might want to check that out.

J. B.

Turbo4whl · Apr 27, 2026

Does the new thermostat have an air bleed hole? Hot air will not open the thermostat.

Gander · Apr 27, 2026

If you replaced the EGR valve you need to make sure you got a correct replacement. I bought EGR valves twice from 2 separate parts houses that ended up being incorrect. They’d bolt up, but the truck ran like crap once warmed up giving it gas. Turns out neither of them were for my application. Ended up finding a made in the USA Delco EGR valve on eBay. Worked like a charm. Now, I don’t think the EGR valve is the cause of your motor getting hot, but just throwing that out there.

hot and rough, running that is

GTManley

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YakkoWarner

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goldpack

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GTManley

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goldpack

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YakkoWarner

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JBswth

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