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Old 08-09-2010, 02:45 PM
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1988 GM TBI troubleshooting help for hot engine stumbling problem.....

Have a 350 tbi in my dually.... Just bought the rig recently. It has newer engine(around 8000 miles on it). I tuned her up and it runs very strong until outside temps get hot and engine is running at max temp around 220 or so and climbing hills(especially loaded) at steady throttle. (we have a lot of hills here) Engine tries to stall, runs out of throttle, stumbles, backfires and wants to quit, and sometimes does. Pull off to side of road open hood let her cool down for a few and away you go till things get hot under hood again. I have to assume this is something electronic since only happens when very hot.(can tbi's vapor lock???)(also should note idle is not smooth) not like it is going to die but definitely not smooth and engine is not steady.(don't know if this has to do with main problem I am having). No trouble codes showing.
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Old 08-09-2010, 04:43 PM
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Quote:
Originally Posted by mightyhammer
Have a 350 tbi in my dually.... Just bought the rig recently. It has newer engine(around 8000 miles on it). I tuned her up and it runs very strong until outside temps get hot and engine is running at max temp around 220 or so and climbing hills(especially loaded) at steady throttle. (we have a lot of hills here) Engine tries to stall, runs out of throttle, stumbles, backfires and wants to quit, and sometimes does. Pull off to side of road open hood let her cool down for a few and away you go till things get hot under hood again. I have to assume this is something electronic since only happens when very hot.(can tbi's vapor lock???)(also should note idle is not smooth) not like it is going to die but definitely not smooth and engine is not steady.(don't know if this has to do with main problem I am having). No trouble codes showing.
220 is way too hot. No, TBI doesn't vapor lock, but there is a temp sensor dedicated to the injection which might be having problems, It's on the coolant return mounted to a bracket.

But 220 is indicative of a failing cooling system, this engine shouldn't be exceeding 200 and really is best kept around 190 which would take a 180 thermostat. But unless your current thermostat is failing; changing to a 180 is unlikely to show a commensurate drop in temp. Here the issue is probably an old and plugged radiator. Simple cooling system cleaners would most likely be ineffective, you're at the point where the old radiator needs to be replaced. You'll find the heater core is in the same condition and should get replaced along with the hoses and maybe the pump. An 88 model is getting rather old at this point.

Bogie
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Old 08-09-2010, 08:09 PM
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I know the sensor that you are speaking of, but there is no sensor on/near return line on this particular rig, not even on thermostat housing.... Engine only runs 220 when climbing very steep hills with 4000 lb camper in bed. Seems to be a reasonable temp given my experience running loaded trucks in hill country when temps hovering over 100 deg. outside. Runs max 190-tops maybe 210 empty still does stumbling thing. Not sure coolant is issue. Engine not overheating but definitely only happens when engine is at max operating temp. and climbing hills with steady throttle pressure.
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Old 08-09-2010, 08:34 PM
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That truck must be fantastic on fuel.

Shot in the dark, bad ignition module. Possibly, probably not, coil.
The dist acts as a heat sink for the module, that is bolted to it.. had a few of them. They can also act like the timing chain is out of it when they crap out.
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Old 08-09-2010, 08:39 PM
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The very first thing I would do is replace the temp sensor for the computer. There is two temp sensors on that engine. There should be one in the drivers side head for the gauge and the one on the intake for the computer. Replace the one on the intake. I believe it is close to the water neck and is a two wire plug. I would say the sensor is telling the ecm the engine is running hot and the ecm is dumping the engine full of fuel to try and cool the engine down. I believe it is this one.

http://shop.advanceautoparts.com/webapp/wcs/stores/servlet/product_Coolant-Temperature-Sensor-BWD_5012767-P_290_R|GRPSENSAMS_575775682___
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Old 08-10-2010, 08:24 AM
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I believe that the temp sensor is heading in the right direction. Can the temp sensors be tested for resistance cold versus hot reading?.... Pretty sure they can. I am just getting into running performance tbi engines and have to admit I am knee deep in the learning curve. I am finally converting from carburetors..... somewhat...lol. Just cant let go of the sound of a dominator sittin' on top of 700 RAT ponies.... You know what i'm talkin about!
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Old 08-10-2010, 11:34 AM
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Ok... Think we hit her with the ecm. I changed out temp sensor and figured for chitz and gigs I would do ecm as well.... Pulled screws and were very corroded almost halfway through, plus a lot of carbon buildup on contacts. Put in new ecm and new screws with some dielectric grease where it needed to be cleaned contacts and it is defiantly running way better. Not very hot today and don't feel like loading camper to test her out. When we give her the acid test will let know.......
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Old 08-10-2010, 11:38 AM
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Coolant Temp Sensor
With engine off key on remove connector, check voltage of yellow wire should be 5 volts.
With connector plugged back in voltage should be high when engine is cold and get lower as engine gets hotter (1.5 - 2.0 volts at 195 degrees).
Also if its disconnected when running computer is fooled into believing its -40 degrees.

Wires
Yellow 5 volts from computer
Black ground


With connector removed ohm check sensor.

C F OHMS
100 212 177
90 194 241
80 176 332
70 158 467
60 140 667
50 122 973
45 113 1118
40 104 1459
35 95 1802
30 86 2238
25 77 2796
20 68 3520
15 59 4450
10 50 5670
5 41 7280
0 32 9420
-5 23 12300
-10 14 16180
-15 5 21450
-20 -4 28680
-30 -22 52700
-40 -40 100700



I think its ignition related myself. Sounds like module is failing when hot.
Maybe dielectric grease is not allowing heat transfer away from module and into distributor base all that good anymore.
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Old 08-10-2010, 11:52 AM
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Thanks for the specs Northstar.... Much appreciated. Must have posted around same time as I posted earlier about ecm.... Goin' campin' again 2 weeks and will definitely know cuz very steep long pull to get up on mountain....lol
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