: MAP Problem?



growe3
04-30-05, 04:12 PM
I think I have a MAP going bad, but I am not sure.

93 STS w/4.6

Symptoms: At just off idle, when accelerating normally I get a slight buck; a little heaver throttle, and a bigger buck.

Very slight surging at 30 MPH or lower, at higher speeds seems fine. WOT is fine.

It is very reminiscent of a carburetor with an accelerator pump going/gone bad.

Spark plugs, spark plug wires and coils are fine. I swapped wires and coil with a known good set and no change in symptoms. Plugs are fresh AC Delco 41-950.

Idle normal, with slight roughness at times.

No error codes have been set.

FPR replaced three months ago.

Checked diagnostics and the MAP seems within limits, but the symptoms seem to point at it as the problem.

Any ideas or suggestions?

-George

eldorado1
04-30-05, 04:49 PM
I'd suspect the TPS before the MAP... MAP sensors don't go bad very often. TPS' are more of a "wear item". I'm sure you can test it via the instrument cluster, or the old fashioned way with a voltmeter... Check for any spikes, or dips in the reading when you smoothly apply throttle with the key on, engine off.

TPS' ALWAYS wear around the light-medium throttle area, because that's where most of the time is spent. I think if you had it backfire and chugg when you floored it, THEN you could suspect the MAP.

BeelzeBob
04-30-05, 06:14 PM
My bet is on fuel pressure. Get a fuel pressure gauge on it and drive it to see what the fuel pressure does when the driveability problem manifests itself.

growe3
05-01-05, 11:14 AM
Thanks for the replies, I will will be checking out both conditions soon.

-George

growe3
05-03-05, 10:51 PM
This a cross-post update.

Results:
Start and idle = 37-38 PSI
Just starting to apply throttle = slight dip to 36 PSI then back to 39 PSI
At about 1500 RPMs = 38-39 PSI (bumped 40 PSI momentarily)

1. What do you think, is this enough pressure to operate the injectors properly?

2. Does the slight 3-4 PSI fluctuation seem normal?

If I need a fuel pump so be it, but I hate to go through the cost and labor if the pressure is normal. So far I havenít found the fuel pressure specification in the manual.

-George

BeelzeBob
05-03-05, 10:59 PM
Was that driving with load on the engine or just blipping the throttle while idling?? You really need to drive it and put load on the engine and see what the fuel pressure does when the hesitation occurs. I usually would just put the guage on and go do some WOT accels. That puts the highest demand on the fuel pump and if it is getting weak you will see the pressure drop off as the RPM's rise...it should stay perfectly solid once you hit the throttle.

growe3
05-03-05, 11:07 PM
That was with the car stationary. I was careful to try and give it the gas similar to as driving. Not just hammer it.

There also felt like a slight miss at idle and low RPM. Today when coming up on of the hills at about 65 MPH in D, it started chugging. I dropped it to 3rd and maintained speed and it smoothed out.

It doesn't seem to like a light load with low throttle angle. If I am right, that is when the engine needs the extra fuel to make good power, as opposed to just staying running.

Mileage stays pretty consistant at 21.6+ average, mostly highway driving.

This condition started coming on over the last couple of months or so.

BeelzeBob
05-03-05, 11:26 PM
Put the gauge on the rail, tape it to the hood and go for a drive...the only way to tell is to put the engine under load to see if the fuel pressure stays up.

eldorado1
05-03-05, 11:51 PM
This a cross-post update.

Results:
Start and idle = 37-38 PSI
Just starting to apply throttle = slight dip to 36 PSI then back to 39 PSI
At about 1500 RPMs = 38-39 PSI (bumped 40 PSI momentarily)

1. What do you think, is this enough pressure to operate the injectors properly?

2. Does the slight 3-4 PSI fluctuation seem normal?

If I need a fuel pump so be it, but I hate to go through the cost and labor if the pressure is normal. So far I havenít found the fuel pressure specification in the manual.

-George

IIRC my fuel pressure is around 40 @ idle... Raises about 4psi at WOT. If it does turn out to be a fuel pressure related item, replace the fuel filter. It just takes one bad tank to clog one up.... But that normally doesn't affect the idle, but moderate to heavy throttle. (loss of power... bucking... hesitation...)

growe3
05-03-05, 11:56 PM
IIRC my fuel pressure is around 40 @ idle... Raises about 4psi at WOT. If it does turn out to be a fuel pressure related item, replace the fuel filter. It just takes one bad tank to clog one up.... But that normally doesn't affect the idle, but moderate to heavy throttle. (loss of power... bucking... hesitation...)

Thanks for the reply, that gives me a little something to compare with. I will try to do another test tomorrow, weather permitting.

-George

growe3
05-07-05, 03:06 PM
I decided to replace my fuel filter before going any farther in testing.
I removed the one-year-old filter and cut it open; to see if any unusual junk was inside, nope it was just fine.

When I was putting on the new filter for some reason the plastic filter holder was being a bit difficult. This caused me to get a light and look closer. With the light on I pressed the fuel filter onto the front fuel line, I then pushed on the rear fuel line.

Whoa there!!! :hmm: The rear fuel line was partially kinked!

As everything was positioned correctly I examined closer and found that the rear fuel line was too long; so that when connected to the fuel filter it kinked the line. I disconnected the rear line, manipulated it higher toward the fuel tank, and then reconnected it. No more kink, the line still looks good.

Summary:
The kinked line would explain the fuel starvation feeling I was getting and the roughness when just starting to accelerate. I am not quite sure how it happened. The filter has been on for about a year, the roughness and stumble just started about two months ago, on an intermittent basis. I am guessing that I pulled the fuel line down too much when I replaced the filter last time, and that the rear fuel line was under, out of plane compression. Over time the line finally kinked.

New readings below: :)
Start and idle = 39-40 PSI
Just starting to apply throttle = quick jump to 47-48 PSI then back to 40 PSI
At about 1500 RPMs = 40-42 PSI

Bbobynski :coolgleam was correct in thinking that the root of the problem was fuel pressure related.

If anyone else has similar symptoms, it may pay to take a close look to be sure that your fuel lines do not have a kink(s) near the fuel filter connections.

-George

eldorado1
05-07-05, 03:35 PM
So does this mean problem solved? Does it work now?

Good job. I like simple solutions.

growe3
05-07-05, 08:58 PM
So does this mean problem solved? Does it work now?

Good job. I like simple solutions.

Yep, problem is solved. Runs great.

It actually ran fine until lately. I didn't see any evidence of any road debris hitting the filter, so it must have been my fault for not making sure the fuel line was in a relaxed position after I installed it last year.

I will be more careful in the future.

-George

BeelzeBob
05-09-05, 12:53 AM
Glad you found the problem!!!

That is why fuel pressure gauges are a staple in solving driveability problems like that. It is also why the fuel rails always have a schraeder valve on them for checking the fuel pressure. I costs quite a bit to put that on every car but it is a fundamental check that needs to be made if there is a problem and hacking into a high pressure fuel line is not easy and not something that we would endorse...

Possibly the line was misplaced earlier like you suspect and in the colder weather the plastic line was rigid enough to not kink. As the weater warmed up and the tubing was subjected to higher temps it got a little softer and collapsed/kinked due to the physical pressure on it from the misposition.