MAP Sensor replacement not solving engine issue

[NotEnoughGPs]

Pontiac Grand Prix GTP, 187,000 miles. All stock except for K&N air filter.

I got a good price on this Limited Edition GP which was bought new by an older guy. I have fixed some issues with it, but the main issue which the original owner complained about is still occurring. I need everyone's input on what to do next.

 

The engine will fail to start at times, and it seems to be completely random. When I first drove it home, I shut off the engine, and seconds later, it would not restart. Then, next day, I installed a new MAF sensor and for a while, the engine started every time for awhile, but then there were times it would not start, either when engine was cold or completely warmed up.

 

When the engine does start when cold or warm, it idles normally. No sputtering and the tach needle stays steady as can be.

 

The check engine light comes on normally at start up, but it has not come one while the engine is running.

 

I replaced the fuel pump with a known working pump as the fuel level sensor was bad on the old pump. And I was told that the pump could be causing the problems with starting. But the fuel pressure was 45PSI in the pump I installed.

When driving 15 miles or more, I noticed that the stock boost gauge on the DIC started to register boost even when not touching the gas, and after keep on driving, it would show full boost off and on even at idle.

 

I found out that the MAP sensor is what sends the signal to the boost gauge. I went about replacing the MAP sensor, and on a drive up to 15 miles, the car ran normally, but then slowly I could see boost registering on the gauge. And then the engine seems to lose power and the idle was erratic. I shut off the engine, and it would not restart right way. FInally, after 20 minutes of trying, the engine started again.

This happened earlier today.

 

There has been times that even with car sitting and completely cooled down, it will not start. At one point, I thought maybe it was starved for air. I removed the air cleaner and it was quite dirty, with bugs and grass all through it. I cleaned this filter and when I went to start the car again, it started right up after numerous attempts before cleaning the filter.

But after the experience today, I can't trust taking the car anywhere far.

 

I have not had it checked for codes yet, but tomorrow I may head to a Autozone where a guy there has helped me in the past on other GP's.

 

I have carefully checked the wires heading to the MAP sensor and I don't see broken wires. Also, the vacuum hose and fitting was in good condition. 

I have also checked all vacuum hoses to see if there are any bad ones, they all look okay.

 

Is there an electrical problem with the harness leading to the MAP sensor? Or is something wrong with the vacuum?

 

Also, I have checked the exhaust and it looks to me that the original catalytic converter is in place on this car. 

Also, I suspect that the original O2 sensors are in place. 

[wstefan20]

Hm... Let us know if you find any codes. I dealt with something similar on my L36 with the maf sensor. I changed it three times with brand new maf sensors. Scottydogs over at the old gpf recommended picking up a used oem maf sensor. I was driving a different car by the end of the day! Not saying this will absolutely work for you, but myself and quite a few other gp owners at the gpf had a rant about this recently and there are plenty of stories how aftermarket maf sensor are horrible. 

 

I'd also check the vacuum lines since they feed the map sensor directly on the supercharged engines. A vacuum leak could cause these symptoms. I usually check with a propane torch with a piece of fuel line attached (with engine cold) and hit all the high risk vacuum locations to see if rpm changes to pinpoint leaks. (just be very, very careful and have a fire extinguisher just in case)

[Galaxie500XL]

Could be a bad crankshaft position sensor...not terribly uncommon, and it wouldn’t set a code at all.

[digitaloutsider]

While I agree that people should only use OEM/Hitachi MAF sensors, unfortunately neither that nor the crank sensor explain the car "seeing" boost when there isn't any.

 

I think your next step is backprobing the wiring from the MAP to the PCM with a multimeter and making sure there is proper continuity.  I'd have to check the service manual to find the other details.

[Imp558]

In the end it's going to be best to get it on a scan tool. It may have a pending code stored to guide you and if it doesn't it really should see a tool that can see live data or take log files.

[Dark Ride]

Check codes first is my thought as well, but having replaced the map sensor and confirmed the wiring, I think next I would be checking that the grounds are good and that the harness leading to the pcm is in good shape. This sounds to me like some sort of electrical gremlin.

[Imp558]

Well, the MAP gets 5v and ground right out of the PCM. I concur verifying grounds and a good visual inspection are a great idea in general but grounds usually result in multiple failures. In 2017 it really makes sense for anyone that maintains a vehicle to have a capable tool. ELM interfaces are under $10 on Ebay and such.

[Schurkey]

The engine will fail to start at times,

it would not restart.

it would not start, 

it would not restart right way. FInally, after 20 minutes of trying, the engine started again.

it will not start.

There's a huge difference in diagnosis between "Won't Crank" and "Cranks but won't run".  Both involve the engine not starting.  Which is it?

 

 

 

 

How old are the spark plugs?

 

Agree:  This vehicle and a scan tool need to meet each other.

[Galaxie500XL]

While I agree that people should only use OEM/Hitachi MAF sensors, unfortunately neither that nor the crank sensor explain the car "seeing" boost when there isn't any.

 

I think your next step is backprobing the wiring from the MAP to the PCM with a multimeter and making sure there is proper continuity.  I'd have to check the service manual to find the other details.

I probably confused the situation a bit...there's two conversations going on at the same time, one for the boost gauge issue, and a secondary topic about the car not starting at times...and I was referring to the start issue.

 

Sorry for the confusion..

[White93z34]

If I'm reading this right the core issue is that sometimes it won't start

 

I'm not entirely convinced this is a MAP/MAF sensor issue. 

 

Unfortunately, in this era of garbage replacement parts you may or may not have introduced issues by replacing both of those. Its hard to say.

 

I'd start with the basics, Spark, Air Fuel

 

And the car will run without a MAF so if that is suspect simply unplug it, you'll get a SES lamp but whatever

[Nas Escobar]

I replied on the other thread about this and mentioned the sensors that regulate the air and fuel (TPS, IAC, MAF), but considering that it has a used fuel pump, it would be a good idea to test the pressure of the fuel pump at prime and see if it leaks down, then check the pressure when the car is running (once it starts). I had to swap my fuel pump twice because the crap carquest pump died 2 months after installing it so a dying fuel pump even though it's "freshly installed" isn't unheard of.

 

It would be smart to check the computer with a scan tool and see if there's pending codes.

[wstefan20]

I'd wager there's probably a code for the MAP sensor. A vacuum leak could cause the rough idle and stalling you described. I'd check that first. For good measure I'd troubleshoot the MAP circuit just to be sure. It has 5v ground and signal all labeled on the sensor.

 

It think you're dealing with two somewhat separate issues though. First, I'd get the codes scanned. Then go from there. If it is the fuel pump, it'll be very hard to pin down if it's intermittent, but I'd check the fuel pressure just to be safe.

[mfewtrail]

There's a huge difference in diagnosis between "Won't Crank" and "Cranks but won't run".  Both involve the engine not starting.  Which is it?

 

 

 

 

How old are the spark plugs?

 

Agree:  This vehicle and a scan tool need to meet each other.

 

Considering what he typed, it appears to be a "crank, no start" when it will not actually start.

 

 

Besides checking for stored or pending codes that aren't currently illuminating the check engine light, I would be checking for both fuel and spark when it fails to start. Both of those things should lead you in the right direction. When you try those things, let us know the results.

Edited December 8, 2017 by mfewtrail

[NotEnoughGPs]

Hm... Let us know if you find any codes. I dealt with something similar on my L36 with the maf sensor. I changed it three times with brand new maf sensors. Scottydogs over at the old gpf recommended picking up a used oem maf sensor. I was driving a different car by the end of the day! Not saying this will absolutely work for you, but myself and quite a few other gp owners at the gpf had a rant about this recently and there are plenty of stories how aftermarket maf sensor are horrible. 

 

I'd also check the vacuum lines since they feed the map sensor directly on the supercharged engines. A vacuum leak could cause these symptoms. I usually check with a propane torch with a piece of fuel line attached (with engine cold) and hit all the high risk vacuum locations to see if rpm changes to pinpoint leaks. (just be very, very careful and have a fire extinguisher just in case)

I am using a OEM MAF sensor, one that I bought brand new and I had used it in another GP this past summer and had no issues with it. I totally agree about aftermarket MAF's being junk as I have already discovered the hard way that they can be suspect.

This issue I am having does act as though it is the MAF sensor gone bad, also due to the fact that the check engine light has not turned on.

But it is the boost that is showing up on the boost gauge that is the beginnings of the car running rough and then after engine shutdown, it will not restart again for 20 minutes or more.

 

As for the vacuum lines, many of them appear brittle to me. I hate to see that. I have quite a bit of brand new vacuum lines as well as a kit full of all sorts of fittings, elbows, T's, and other connectors. 

This weekend, I plan to replace as much of the lines as I can, whether it needs it or not.

Of course, I want to focus on the vacuum line and the connectors going to the MAP sensor. 

I have googled "MAP sensor" and read through dozens of articles on other GM car forums and found out that the MAP sensor can lead to problems with the engine not starting or poor performance. I believe that brittle vacuum lines can be a cause of this also. I have heard of others trying the propane torch method, but I am not that brave. 

[Imp558]

Well, you don't actually LIGHT the torch. The propane test has never worked for me regardless. You have a heat soak issue ggoing on too which is usually CKP (oh you're going to love changing that one when the time comes) or an ICM. For the first time this year I saw heat soak with a PCM. theoretically you could have vapor lock but while I've heard tales of it I've never seen it first hand so always personally wrote it off as something people say when they can't figure out the real problem.

[digitaloutsider]

I'm not buying this being any other issue than a bad MAP, MAP wiring, or PCM. 

 

Robert, grab a scantool and check out what's going on with the MAP when this happens. Otherwise we're just chasing ghosts.

[Imp558]

This summer on my way to the gathering Sam and I had to pull over and fix vacuum lines on the Monte, they were 3 days old.

[wstefan20]

I'm not buying this being any other issue than a bad MAP, MAP wiring, or PCM. 

 

Robert, grab a scantool and check out what's going on with the MAP when this happens. Otherwise we're just chasing ghosts.

 

Agreed. Though if it comes to it, or anyone stumbles upon this thread needing to change the CPS, (I'll have to post a separate thread eventually about this) I bought three balancer pullers before I found this: 

 

https://www.autozone.com/loan-a-tools/harmonic-balancer-installer/oem-harmonic-balancer-puller-retrofit-kit-for-gm/409642_0_0

 

They also rent these at all autozones. It works with the autozone loan tool balancer pullers and is the only rental tool that works to pull our pulley's at any of the local auto parts stores. 

 

Also as a quick tip for either A) those who don't like the starter trick to spin the balance bolt off or those who can't crank their car over at the time (aka have stuff off or junkyard) the easiest way I've found is taking a jack stand and taping a flat-blade screw driver to it and using this "tool" to rest against where the flywheel inspection cover goes and in the flywheel teeth. Don't worry, I've done this dozens of times and have not once damaged either the transmission housing, flywheel teeth, or the tool! This method works really well for installation as well!

 

Hope this helps someone and hopefully you don't have to replace yours. 

 

Again, post those codes!!!

[vipmiller803]

I've used a large two jaw puller with success many times. I never could get the right bolt thread on my balancer when I needed it.

 

An impact never failed me on a crank bolt but vice grips around the flex plate plus a long breaker bar also works.

[wstefan20]

I've used a large two jaw puller with success many times. I never could get the right bolt thread on my balancer when I needed it.

 

An impact never failed me on a crank bolt but vice grips around the flex plate plus a long breaker bar also works.

 

Forgot which model engine we were talking about here! lol ignore my prior post unless you have a 3800 about the puller bolts!

 

To clarify, any of the "solid" harmonic dampers you can use a regular puller tool, but you do not want to use one on the 3800s since they are a two piece rubber and I've personally ripped them apart using a regular puller. 

 

Haven't tried using vice grips though. That could work well!

 

Wish I had an impact and wish I could use one at the junkyard!  

[NotEnoughGPs]

There's a huge difference in diagnosis between "Won't Crank" and "Cranks but won't run".  Both involve the engine not starting.  Which is it?

 

 

 

 

How old are the spark plugs?

 

Agree:  This vehicle and a scan tool need to meet each other.

Yes, about the long winded description of mine, I need to clarify that this 2003 GTP has an "Engine cranks but won't start" condition. It won't start after the engine has been driven 15 miles or more.

The spark plugs and wires were changed out by the previous owner already. The is no CEL on, and the car starts up easily on a cold engine, so I don't suspect there is any issue with spark. 

I do plan to get a good scan tool on it, but I have to drive over 35 miles to have this done. Right now, I am hoping to fix the no-start condition by replacing vacuum hoses, and the catalytic converter, and the wiring leading to the MAP sensor.

[NotEnoughGPs]

Agreed. Though if it comes to it, or anyone stumbles upon this thread needing to change the CPS, (I'll have to post a separate thread eventually about this) I bought three balancer pullers before I found this: 

 

https://www.autozone.com/loan-a-tools/harmonic-balancer-installer/oem-harmonic-balancer-puller-retrofit-kit-for-gm/409642_0_0

 

They also rent these at all autozones. It works with the autozone loan tool balancer pullers and is the only rental tool that works to pull our pulley's at any of the local auto parts stores. 

 

Also as a quick tip for either A) those who don't like the starter trick to spin the balance bolt off or those who can't crank their car over at the time (aka have stuff off or junkyard) the easiest way I've found is taking a jack stand and taping a flat-blade screw driver to it and using this "tool" to rest against where the flywheel inspection cover goes and in the flywheel teeth. Don't worry, I've done this dozens of times and have not once damaged either the transmission housing, flywheel teeth, or the tool! This method works really well for installation as well!

 

Hope this helps someone and hopefully you don't have to replace yours. 

 

Again, post those codes!!!

From extensive reading on the CPS, I believe that it is not to blame for the no-start condition. The CPS does not get heatsoaked according to everything I have read. When a CPS is faulty, it can cause the engine to stop at any time. Also, I was asked to observe the RPM's on the tach when cranking. If the CPS is bad, then the tach will be sitting at zero . That is my understanding. 

As for a scan tool diagnosis, that is going to occur soon.

[wstefan20]

Ok. Got a bit confused in the initial description. Sounds like a circuit is failing when warmed up. Definitely could be the fuel pump. I hate to say it, but the best way to troubleshoot something like this with no codes is to get a good scan-tool with data logging ability. I'd recommend the launch easydiag 2.0 plus that I've used for over a year and I love it, but unfortunately the price jumped up from $70 to $150 and you can't buy it on amazon anymore. I'm sure others will jump in to name their favorite tool.

 

I wouldn't go re-wiring the map sensor unless you've confirmed that the circuit is a fault. A cheap mulitmeter will confirm this.

[wstefan20]

From extensive reading on the CPS, I believe that it is not to blame for the no-start condition. The CPS does not get heatsoaked according to everything I have read. When a CPS is faulty, it can cause the engine to stop at any time. Also, I was asked to observe the RPM's on the tach when cranking. If the CPS is bad, then the tach will be sitting at zero . That is my understanding. 

As for a scan tool diagnosis, that is going to occur soon.

I agree with your conclusion on the CPS, though I've seen some weird stuff, I don't think this is to blame here.

[NotEnoughGPs]

UPDATE: I am still trying in my spare time to work on this GTP. 

In the past week, I have had no trouble getting it started on a cold engine. Now, that has changed again, I tried to start it to move it into the garage and it would not start after 8 attempts. It cranked, but would not start, a few times it almost started. A few hours later, I tried starting it again and it started right up.

 

I had stopped at a shop few days ago about 5 miles away and talked with a mechanic who is recommended by some other local people I know. I explained to him what has been happening with the GTP. I told him I wanted to install the exhaust with the low mileage catalytic converter, and replace the rear valve cover gasket, and some other items. 

I told him that I may be able to find what is causing the car to not start, and he said if I can't to call and make appointment and leave the car. He said he would check the fuel pressure gauge, and hook up a scan tool to check for codes, and he would check wires with a ohmmeter, he has done that before.

 

Anyways, last night, when I googled "3800 engine cranks but no start", one owner said he had a problem with the ICM (Ignition Control Module). He removed it and the underside was severely corroded. I had never had the coils or ICM removed from this car (actually never removed it from any GP I have owned) 

When I did take off the ICM, it was severely corroded, both the module and the back plate. I removed the corrosion by had with various grades of sandpaper until the surface was shiny.

 

After putting on a coat of dielectric grease, and carefully putting everything back together, I turned the key to start and without hesitation, the engine started up.

 

I did not replace the ICM, I have other ones in the garage, but now I am not going to replace the one in the car.

I want to take it for a long drive and see if the boost gauge shows the level of boost as it was before.

 

I will report back again soon.