MAP Sensor. If it's bad, what happens?


BoxMasters
09-04-2004, 06:06 PM
What happens if the MAP sensor is bad on my 1991 2500 454 TBI Suburban? What effects will be noticed.

The local shop said they replaced it, but it doesn't look new. His hands were always dirty, but the MAP sensor looks old. He could not ID the problem.

I've got a miss and hessitation when accelerating.
He said he replaced the MAP. but it didn't help, now I'm worried I'm replacing parts and my have the original MAP
Since then I've replaced the entire distributor, Ignition coil, new spark plugs and new wires, PCV valve, TPS sensor,Fuel filter,Oxygen Sensor, and EGRvlave.
The 454 used to fire up and idle high till warm, it always did since new, but last month after a trip it started missing. He couldn't ID the problem, so I started replacing the things that may cause the problem, but it's still there

SpitAndDirt
09-05-2004, 03:22 AM
Any codes? Go get this book if you REALLY want to understand all the engine management electronics in your truck: How to Tune & Modify Chevrolet Fuel Injection (Powertech)
by Ben Watson It's sold by Books-A-Million, Barnes and Noble, and Borders. Not to mention Amazon.com and others. You will be able to troubleshoot your problem better than your mechanic after you skim through the book. Trust me. -Chris

GMMerlin
09-05-2004, 06:14 AM
A faulty MAP sensor can cause some fuel delivery problems and not turn on the SES light.
Does your misfire happen at idle or while driving?
If it happens at idle, you can remove 1 plug wire at a time to identify the effected cylinder.
Many things can cause a misfire, low compression, secondary ignition, fuel delivery

BoxMasters
09-08-2004, 08:51 AM
Fuel is seen spraying in a cone shape from the TBI injectors, the timing is correct, I can pull each plug wire off one at a time and the engine faulters some. For 130K miles the 454 would race up at cold startup, and idle back down slowly after say two minutes.
One thing I noticed about 6 months ago, it always started as soon as I turned the key, I not sure if it made 1 revolution and it started. It would almost start as soon as I put the key in, but it started having to turn over two times, then three times, then four. Un like since new, it had to make several turns before starting , then the hessitation started, then the missing started.
No error codes, no SES lite. It runs but poorly.
I'm thinking about asking the parts place if Ican return a new MAP sensor if my old one is not bad.
The slow start and the low idle at cold start were the first conditions leading to today.

Fireplug
09-14-2004, 05:43 PM
If the MAP was bad it will set the mil light(1991 check engine light name changed to mil malfunction indicator lamp)
If you do not have a fast idle at start up then you might have a coolant temp sensor going bad.
Its easy to check that with a scan tool and engine cold. If the sensor fails in the cold mode the ecm thinks the engine is warmer than it is and leans out the system
It will run like crap hot or cold because even thow the mil light is not on the ecm is in open loop also the tbi injectors are known to go bad. They might look like they are spraying ok but 1 is just a little weaker then the other.
I would check the coolant temp sensor first
Fireplug

BoxMasters
09-18-2004, 12:53 PM
Hi, I changed the Engine Coolant Temp Sensor last night, and let the tread sealer set over night, this morning I refilled the antifreeze and reconnected the battery, and started the engine, but it still did not have the high idle as it used to on a cold start, and the miss and hessitation is still there. I don't mind changing the small stuff, and I'm saving the old parts, trying to keep from taking the truck to the dealer right now. I really think the problem is related to the emissions garbage.
I wish my 454 was as simple to work on as my 1952 farm tractor.

Any more ideas would be helpful, my wrench is in my hand ready to change anothe part, I just don't know which one I should try next.
Thanks again

aBieker
09-21-2004, 09:01 PM
I have a similar problem that just started on my '95 Millenia. I had my timing get off when my belt skipped 9 teeth (whole other story). So, I reset the timing (which is now correct) and all the sudden, my car is missing OCCASIONALLY. No cycle. No pattern. The only thing I can zero in on is that it happens only when the car gets warm and its onset is accelerated by hard driving and quick acceleration. I have NO idea what is happening. I thought it might be the plug wires (warmer = more resistance = miss in ignition)... but no luck. It did seem to make the phenomenon take longer to occur... but it still does.

Westcm
02-15-2005, 11:48 AM
BoxMasters,

I've been having the same problems! Have you figured it out yet?

Burb95
03-11-2005, 06:35 AM
Boxmasters,

You have been experiencing almost the same symptoms I am dealing with on my 95 Suburban 5.7Liter TBI. I think I am getting closer to the solution. You can read my thread about the details. I thinks one of my sensors is not responding to changes quickly enough so the ECM is not adjusting at the correct time. I am going to replace the oxygen sensor this week. If you replaced the plugs with platinums, you might consider going back to the original coppers. The platinums caused me more problems. Please let me know if you have solved your problem yet. It helps everyone to understand the final solution.


Burb95

84fiero123
03-12-2005, 08:18 AM
unplug the dam thing ( map sensor) and leave it unplugeg the computer will reset itself to the basic settings. if it runs fine replace the sensor if you still have the problem its something else.

DDotson
03-12-2005, 09:05 PM
Just a couple of quick notes. If you don't trust your mechanic at replacing what he says he is, Tell them when you take it in that you want all the old parts that are replaced. Usually if you go to a dealership they can run a diagnostics on it for about 70 bucks that they will deduct from the bill if you have them replace the parts that are bad. Think about how much money and time and aggravation you allready put into buying and replacing parts to no avail. I had a similar problem and found out (by taking it to the dealership) that the EGR valve was slow responding causing all the problems. I do not work for a dealership I just believe in doing things right the first time. They are trained on your vehicle and it codes and responses.

tomboy630
04-07-2010, 07:44 PM
I had the same problem a while back. I had more problems combined with it, but once they had the water pump changed, and the thermostat changed, and then the distributor shaft changed it ran just as you suggested. Hard to get it up to speed, and it used twice as much gas as it usually does. So, I took it to my mechanic... and he said the timing was out.. they had me bring it in and they fixed it and poof it was running good a new.. try that... it's worth a shot.

j cAT
04-07-2010, 08:09 PM
I had the same problem a while back. I had more problems combined with it, but once they had the water pump changed, and the thermostat changed, and then the distributor shaft changed it ran just as you suggested. Hard to get it up to speed, and it used twice as much gas as it usually does. So, I took it to my mechanic... and he said the timing was out.. they had me bring it in and they fixed it and poof it was running good a new.. try that... it's worth a shot.

welcome to the forum..

your reply to a members issue is a little late, like 5yrs late...

read the rules on the digging up old postings ...

youngkia1
04-08-2010, 03:50 AM
[quote=BoxMasters;2209882]What happens if the MAP sensor is bad on my 1991 2500 454 TBI Suburban? What effects will be noticed.

The MAP sensor is mounted on or connected to the intake manifold to monitor intake vacuum. It changes voltage or frequency as manifold pressure changes.

chevytough89
04-13-2010, 11:34 AM
well we have a 89 chevy 1 ton with a 454 with about the same milage and we had the same egzact problem beleve it or not the distributer bushing was wore the best way to diagnose is to take off your distrbuter cap and wiggle your rotor from side to side to se if the distributer shaft has any play it will throw it off center and make it missfire and low rpms.... does the missfire quit when you bring the engine around 2500 rpms??? if so than that is your problem it wont thow a ses code eather wich is a pain

Darthjav
07-02-2013, 08:11 AM
I had to change the Fuel injectors along with the MAP sensor.

No futher problems encountered.

The fuel injectors will give you all sorts of codes and you will be chasing ghosts forever. The old system is awful. New redesigned ones are a vast improvement.

took about an hour to do myself and you can view different YouTube videos on how to do it. Not very difficult at all.

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