[LittleNo]

I've been working on the Northstars at work for many years now, however those 4.9 were before my time (maybe I'm too young for them )

I got a 91 Eldorado with the 4.9 from a buddy because the engine was not starting. First there was no spark, so I gave it a complete tuneup (whole distributor, wires, plugs). Now it fires and starts. However it's running way too rich (you can smell the unburnt fuel from the exhaust and it's not driveable, engine dies with the pedal pushed). I checked for DTCs but there are none (battery was drained because the car sat for almost a year in a garage, so maybe it didn't run long enough for DTCs to set). However, what I can see on the Tech2, the MAP-value is around 90 kpa all the time (I don't have another 4.9 to compare, but on the N*s it's around 30 kpa at idle). I tried another MAP sensor with no change.

So my guess is, there must be a large vacuum leak. It must be much bigger as a disconnected brake booster could cause. Could it be an air leaking intake manifold gasket? or are there any other common places where this engine can pull wrong air?

 

[Ranger]

you can smell the unburnt fuel from the exhaust

Ruptured FPR diaphragm? Stuck open fuel injector? A vacuum leak would cause it to run lean.

 

[LittleNo]

thanks, I checked the FPR, no leaking fuel to the vacuum line there.

OK, I will pull the Fuel Rail and check for a leaking injector.

I have an injection pulse width of about 10 ms at idle, same problem as above I don't have a good car to compare, but my 98 N* has only 3 ms at idle (I'm not sure if it's reliable to compare the 4.9 with values from the 4.6 N*, but I thought that indicates that the PCM commands this fuel amount, so I'm looking for reasons what could force the PCM to do so)

 

[drewsdeville]

However, what I can see on the Tech2, the MAP-value is around 90 kpa all the time

This is a problem. At hot idle, you should be somewhere between 30 and 40kpa.

If it's a constant value no matter what the conditions, I'd suspect a bad sensor, assuming you've checked the harness and vacuum line.

 

[LittleNo]

ok, no leaking injectors, everthing is dry on all injectors with the fuel pump on

Originally Posted by LittleNo However, what I can see on the Tech2, the MAP-value is around 90 kpa all the time



This is a problem. At hot idle, you should be somewhere between 30 and 40kpa.

If it's a constant value no matter what the conditions, I'd suspect a bad sensor, assuming you've checked the harness and vacuum line.

I had the same thoughts, so I connected a vaccuum pump to the line of the MAP-sensor, with the ignition on and observing the MAP-value on the Tech2 I got the vaccuum down to 50 kpa...so basically the sensor and it's connection to the PCM works

oh and BTW: the value of the Coolant Temp seems correct (the engine didn't run since yesterday and it shows coolant temp nearly at room tempereature)

 

[drewsdeville]

ok, no leaking injectors, everthing is dry on all injectors with the fuel pump on


I had the same thoughts, so I connected a vaccuum pump to the line of the MAP-sensor, with the ignition on and observing the MAP-value on the Tech2 I got the vaccuum down to 50 kpa...so basically the sensor and it's connection to the PCM works

oh and BTW: the value of the Coolant Temp seems correct (the engine didn't run since yesterday and it shows coolant temp nearly at room tempereature)

Either way, it's incorrect. You can't go any further down the line until you figure out the problem on the top (MAP sensor). The MAP is high up on the food chain as far as a/f control. Anything you change "below" it won't help you. How much vacuum did you have to apply to get it reading 50kpa?

 

[LittleNo]

I don't know (the vacuum pump is just a cheap plastic stuff for brake bleeding), but I see what you mean, I'll check if I can get a more precisely vacuum pump and repeat this test.

 

[Crudsturbo]

In my opinion if the MAP was bad or you had a vacuum leak you would have a check engine light. I have a 91 STS and had a very similar issue. Turned out to be my ECM. Not saying this is your issue but you need to run some tests. If you have an FSM or Alldata they will give you all of the values you need. Also check the wiring to the injectors, if it has been sitting rodents may have gotten to some wires, old fluids may have eaten away at wiring. That is what I discovered on my but my guess is the shorted wiring caused the ECM to fail. So check from the injectors back and see what you get.

 

[LittleNo]

well, yes, the diagnostic procedures in the FSM mostly start from a DTC, and I have none. The symptom diagnosis only runs thru the ignition system which is working now.

I'm still looking for a vaccuum pump with precise instrument to verify the measured value from the MAP correlates the thruth. But I too thought about the ECM. I will find out. Fortunately they're available for a little money from rockauto.

 

[LittleNo]

OK, now it get's interesting. I made a connection to the A/C Service Vacuum pump and it's manifold. So the reading of the MAP-Sensor on the Tech2 equals exactly the reading on the pressure gauge of the A/C Service Manifold. So the MAP sensor and ECM both works fine.

Now the big question is: why do I have so little vacuum inside the intake manifold? I already disconnected and sealed all vacuum ports of the TB. A vacuum leak big like this could be heard, I think. I'm going to test the compression of all cylinders... but I'm open for your ideas.

 

[Crudsturbo]

And yes with no DTC it is hard to trouble shoot. Is the car drivable? I would drive it until a DTC appears, with that much fuel consumption to smell it I am sure a code will come up. There a bunch of small things on the 4.9 that can cause adverse effects, like the EGR tubes, I would suggest removing the throttle body cleaning it well, removing the EGR and running a wire hanger through the EGR tubes. That is where a lot of vacuum is. Although a lot dont recommend it it is not a bad idea to get some Seafoam into the vacuum ports ehile the car is running. Have you checked the timing? The A B pins of the diagnostic connecter have to be jumped to put it in Set Timing Mode and then it has to be 10 degrees BTDC. I honestly think you have a minor issue causing your foul up and wish you the best of luck finding it. Please keep us posted!

 

[Scroggdog]

I just bought a '91 Eldo cheap with 100k miles. It is in good shape, but running like crap. Today I did complete tune up and checked the compression while I was at it. Cylinder-to-cylinder compression varied 120-140 psi. Let me know if you check yours, because I am interested in the comparison. Tomorrow I will change the fuel filter, see how it runs, and check the DTCs. Let me know if there is anything that you want me to compare for you, but I don't have any diagnostic equipment for this old car.

 

[LittleNo]

Is the car drivable? I would drive it until a DTC appears, with that much fuel consumption to smell it I am sure a code will come up.

usually that's what I would do. But this one is not driveable. Absolutely no power to accelerate.

Let me know if there is anything that you want me to compare for you, but I don't have any diagnostic equipment for this old car.

that would be a good moment to get one no, just kidding, I compared everything with my 98 STS, although there are many differences between the 4.9 and the N*, some things are nearly the same, engine size, fuel injector size, [email protected], so I'm pretty sure my problem is the high MAP/low vacuum. I just didn't found out the reason yet.

I removed the EGR now, it's base was broken in two parts, so the engine definately got a lot of wrong air thru the EGR channels. I temporarily closed the EGR opening of the intake manifold, and it seems it made everything a bit better. I have a MAP of 70-80 kpa at idle now (before I never got under 90) and the engine is running a little little bit better now.

Are there any other vacuum-related parts that can bring in a huge amount of air?

 

[LittleNo]

ok, all cylinders have 150 psi, only #1 has 145, that's fine

stupid idea: broken intake manifold gasket? this engine has the plastic gasket with rubber inserts, i've never heard of a problem with them on a 4.9, but the 3.4 V6 (LA1) has the same design of gaskets and they always have problems with them (the plastic cracks and then the rubber hangs loose)

 

[drewsdeville]

This engine had a fiber type gasket from the factory. If they fail, its usually at the coolant crossovers on each corner of the manifold. That said, I did have an intake gasket leak vacuum on mine, and it caused all kinds of fuel trim trouble on that bank. It would be best to put an actual vacuum gauge on the manifold to see how its actually behaving rather than relying on the slow, electronic display on your scan tool. Is the vacuum really a steady 90kpa? Does the gauge needle bounce around or jiggle?

 

[LittleNo]

ok I'll try to get a better gauge, but, what will it tell me? what do I know when I have a steady vacuum at 90 kpa or not?

 

[LittleNo]

I got the BI***! totally strange:

The intake gasket was very dirty but not the problem, I put everything back together with new gaskets. When I had the intake/TB/Fuel Rails back on the engine, I did a test run before all of the other stuff (alternator, PS pump, covers...) which was successful, it ran absolutely perfect. After finishing everything the same problem was back = running rich, no power etc. Ok, at this point I had no clue what to do anymore. So I started from zero = checked for spark (this time not only with the timing light, but with a spark plug connected to a wire and clamped to good engine ground to verify there is a REAL spark) and there was none. Then followed the whole dizzy procedure, but there was everything ok.

Without any further ideas but knowing from the test-run before that the engine is mechanically good, I just removed step by step what I put back on the engine AFTER the point when it ran fine. After unplugging the alternator I got a perfect spark at the plug and the engine fired up and runs perfect since that.

It turned out to be a short circuit inside the alternator, that destroyed the BCM (I don't know why they did this, but yes, the alternators control wires are connected to the BCM under the glove box on this car) which then pulled the ignition voltage of the whole powertrain management (including the ICM inside the distributor) down to only 5-6 volts.

It had been a very long time that a car took me so damn long to get it running, but finally I made it

 

[caddytom61]

I also have a 1988 Eldo smells rich when it starts but the mechanic says it's running lean this car only has 37980 miles the engine looks like new ALT. car. It runs sluggish no power get a lot of dic codes ED anyone have any idea afraid as someone said here it's to old for most don't want to spend a fortune for them to get up to speed