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Hi wondering if someone could help me out. I was driving home and my car started to surge and sputter when idling at a stop light. Check engine light came on and I immediately limped it into the shop I work at. I hooked up the code reader and got a code 33 ,which corresponds to a malfunction with the MAP sensor. I checked for vacuum leaks from the hose to the throttle body and nothing found. I checked for vacuum leaks everywhere else and found nothing. So I replaced the MAP sensor and didn't change a thing. Still sputtered and idles like crap.
So I grabbed the high dollar Snap-on computer and connected it to the obd 1 port and it said the same code, 33. I started the engine and watched the live data. Voltage is out of spec for the MAP sensor. Thought maybe I got a bad part from the parts store, so I ordered another one, but same result. Supposed to be 1 - 1.5 volts at idle and higher than 4 volts at WOT. Voltage never changed. I'm suspecting ECM may be bad. I cleaned IAC and checked other sensors and they seem to be working properly. Fuel spray seems to be fine but I've heard fuel pump may also be an issue with not having enough pressure. I want any other ideas before I replace the Ecm. And also going to do fuel pressure test tomorrow. Any help would be appreciated.
 

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check voltage change at map, not from the ecm live data.. spec voltage to map?? then voltage out at idle and WOT.
it might be the road the signal travels not the sender..
 

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I followed the map sensor test according to alldata, and it's supposed to be 1 to 1.5 volts idling, and is it was higher than 4. Then I disconnected the connector from the MAP sensor and back probed the connector. There it was supposed to be less than 1 volt and it was still higher than 4 volts. Gonna back probe the ecm connector and see what the voltage output is on that particular circuit from the ECM.
 

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I followed the map sensor test according to alldata, and it's supposed to be 1 to 1.5 volts idling, and is it was higher than 4. Then I disconnected the connector from the MAP sensor and back probed the connector. There it was supposed to be less than 1 volt and it was still higher than 4 volts. Gonna back probe the ecm connector and see what the voltage output is on that particular circuit from the ECM.
is the map getting 5 volts input..? at the map? and are you sure you didn't probe the input thinking it was the output..

probing the ecu output at connector, why, I'd only do that if it's not 5 volts input at the map..
if the 5 volts is getting to the map you already kno what voltage the ecu is outputting in that curcuit..
probing the signal from the map to the ecu at the ecu connector if it's 4 volts out of the map it should be 4 at the ecu..
try unplugging the map and check the voltage at the plug that feeds the map.. then with it still unplugged see if there is voltage in the lead that takes signal from map an carries it to ecu.. with that connector unplugged from the map, the lead that fees back to the ecu should have no voltage, if it does you have a short somewhere..
 

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TBI will throw a code 33 if the engine stalls with the key on or almost stalls. The code will set if the vacuum falls below a certain threshold with the throttle position sensor reading closed. I get that code all the time on my old rock crawling truck that has gm tbi swapped on it if I stall trying to bump over a rock or ledge.

Though a map sensor problem could have caused the stumble I'd be willing to bet it set the code because of the stumble, not stumbling because the code.

Check the fuel pressure.
 

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Discussion Starter #6
Someone else also suggested that I checked the fuel pressure, but there isn't rubber line to splice into, it's all steel line or hard plastic. It is also running rich if that helps. I was thinking if I could cut a piece of the steel line so I can get a fuel pressure reading, but don't want to have to replace it with rubber hose and clamps.
 

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A fuel pressure gauge kit, Snap On, Matco, Harbor Freight, etc., has them. You remove the line large line from the rear of the TBI, install the 't' fitting inline and hookup the fuel pressure gauge to the 't' fitting. It's the only way I know how to check the FP on a TBI system. Napa might have one also, but I picked up the Harbor Freight kit for about $45 and now I can check fuel pressure on most any OBD 1 or 2 system.

Good luck - Jim
 

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Sounds like a short in the wiring or a defective ECM.
Ohm the individual wires, with the components in question disconnected of course, to determine if there is a short.Or try another ECM.
 

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Discussion Starter #9
I wanted to try another ecm but with electronic parts, you can't return em if you've used em. I tried getting fuel pressure, but they are metal lines, so there's no way I can get an tbi adapter in there. I dont think fuel pressure is an issue bc when it does run, it runs rich.
 

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Did a fuel pressure test, and it's at 13 psi. Leaning more towards electrical issue now
You really need to concentrate on the code 33 for the map sensor, as this sounds like your problem

bc when it does run, it runs rich.
if you are getting 4 volts from the map in the data stream the ecm will dump fuel to the point it will miss and run like crap.

First check to see if you have vacuum at an idle at the map sensor, these hoses break all the time. Map STILL PLUGGED IN and motor running, back probe it for voltage, (I forget which wire check your flow chart) the voltage should be 1.5 volt ish to 2.5 volt not 4-5 volts if you have vacuum.

report back
regards
 

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Discussion Starter #12
Sorry I haven't replied back sooner, but she's back running like a champ. I wanted to get it back home so it didn't have to sit at the shop so I can do more tests. And so I'm limping it home and about a mile away from my house it starts running like normal again. I'm wondering if it was in limp mode and needed to be driven some to get it out of that mode. While it was at the shop, I replaced fuel filter, MAP sensor, and cleaned the throttle body so I'm wondering if the map sensor was even at fault. Thanks to all you guys for the help.
 

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Hi wondering if someone could help me out. I was driving home and my car started to surge and sputter when idling at a stop light. Check engine light came on and I immediately limped it into the shop I work at. I hooked up the code reader and got a code 33 ,which corresponds to a malfunction with the MAP sensor. I checked for vacuum leaks from the hose to the throttle body and nothing found. I checked for vacuum leaks everywhere else and found nothing. So I replaced the MAP sensor and didn't change a thing. Still sputtered and idles like crap.
So I grabbed the high dollar Snap-on computer and connected it to the obd 1 port and it said the same code, 33. I started the engine and watched the live data. Voltage is out of spec for the MAP sensor. Thought maybe I got a bad part from the parts store, so I ordered another one, but same result. Supposed to be 1 - 1.5 volts at idle and higher than 4 volts at WOT. Voltage never changed. I'm suspecting ECM may be bad. I cleaned IAC and checked other sensors and they seem to be working properly. Fuel spray seems to be fine but I've heard fuel pump may also be an issue with not having enough pressure. I want any other ideas before I replace the Ecm. And also going to do fuel pressure test tomorrow. Any help would be appreciated.
Check your battery and alternator output before spending more money. If either or both are weak it will mess with the MAP sensor and trip the 33 code.

Bogie
 
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