Tried three different MAP sensors, but 2.5 engine still bogs badly when accelerated (Page 1/5)
Patrick JAN 26, 11:33 PM

Fortunately I have another Fiero to drive, but my trusty '84 5-spd is acting up. I noticed it started running very rough when first started. It will improve somewhat after running for a couple of minutes, but something is still screwey. The engine will rev high and stay there easy enough with good power (that indicates to me that the ignition system and fuel supply are probably fine), but the engine will stall unless it's acceleratred very slowly.

There are no trouble codes.

If I disconnect the MAP sensor's electrical connection, the engine will idle and rev up just fine... no bogging.

There is plenty of vacuum (22 in Hg) from the TB to the MAP at idle.

I figured it must be the MAP sensor then. Tried one from another Fiero. Back to the bogging again.

Tried a MAP sensor from a third Fiero. Back to the bogging again.

I doubt that all three of these MAP sensors are faulty.

I haven't checked the voltage readings at the MAP yet, but with what I've reported so far, has anyone got any ideas?

(In case it might have any bearing on suggestions, this engine has always had rather poor gas mileage. Never been above 25 MPG. Kind of lousy for a duke with a 5-spd.)
phonedawgz JAN 27, 07:31 AM
I would suspect low fuel pressure under load.
KurtAKX JAN 27, 08:10 AM

quote
Originally posted by phonedawgz:

I would suspect low fuel pressure under load.



^ What he said. I've had two do the exact same thing, one with a leak in the rubber coupler tube between the pump and sender, and one with a bad pump.

If you pick up a TBI fuel pressure test adapter and a fuel pressure gauge with a hose, you can temporarily tape the gauge to your back window. I bet your fuel pressure under hard acceleration drops down below 7 psi.
conan469 JAN 27, 08:21 AM
i agree with fuel pressure. cheap test would be to pinch of the return line (rubber hose somewhere?) and drive it.
don't leave it clamped for long time.
have done this to get cars back to the shop.

could also be ignition module. have had them get weak and still run.
replaced one few months back, after car died, thern customer came back to thank me for the power increase while driving,
he said it had been bogging down and slow for a while, then it finally died.

remember under load cylinder pressure rises A LOT higher than in neutral.
fuel demands are higher and more spark is needed.

[This message has been edited by conan469 (edited 01-27-2014).]

armos JAN 27, 11:05 AM
I would want to check the fuel pressure just on principle (it's certainly important and it's good to know the info), but I don't think that can explain why it accelerates better when the MAP is unplugged. Unless it's only a minor difference, but it sounds like the change was significant.
I'm confused by the statement that it makes good power, but stalls unless you accelerate slowly. That seems like a contradiction - do you mean it makes good power when the MAP is unplugged?

Does it seem that colder weather corresponds to when the cold idle issue surfaced?

When the MAP is unplugged, the ECM will run more rich. I've seen this when datalogging a V6 car, and I assume the 4cyl has similar programming. That rich condition might be helping to compensate for whatever the problem is. Perhaps the mixture is truly lean for some reason.
Or perhaps the mixture is good, but it's a marginal ignition problem, such that it breaks down under load unless it's running extra rich.

Have you investigated the TPS? A bad TPS could easily cause a lean stumble when accelerating, but I don't know if it can be bad enough to cause a stall. The ECM's logic for the check engine light is really simplistic and can easily miss a fault.
This doesn't really fit the idling problem though - a bad TPS shouldn't affect idle as far as I know.

If you have a meter then definitely check the voltages at the MAP sensor connector. I'd also probably change plugs and check compression while you have them out.
Boostdreamer JAN 27, 11:48 AM
Checked the timing? Power off idle is greatly affected by the timing.

Jonathan
AL87 JAN 27, 12:08 PM
My question here is: What would cause the fuel pressure to drop or remain consistent (but not enough for the demand)?

bad fuel pump? low charge on battery? bad regulator? (if a 2.5 has one)
Csjag JAN 27, 12:13 PM
Is the fuel filter fairly new? Is the exhaust system flowing freely without any obstructions like a clogged cat?
Patrick JAN 27, 02:22 PM
Lots of questions and suggestions to respond to, but I'm just going out the door. I'll touch upon the following before I have to go...


quote
Originally posted by armos:

I would want to check the fuel pressure just on principle (it's certainly important and it's good to know the info), but I don't think that can explain why it accelerates better when the MAP is unplugged. Unless it's only a minor difference, but it sounds like the change was significant.

I'm confused by the statement that it makes good power, but stalls unless you accelerate slowly. That seems like a contradiction - do you mean it makes good power when the MAP is unplugged?



The change is very significant with the MAP unplugged.

With the MAP still connected... If the throttle is cracked open quickly, the engine will stumble badly and sound like it's running on two cylinders. However, if the throttle is opened very slowly, the engine can reach and maintain a high RPM and it sounds fine. This is what I was describing below.


quote
Originally posted by Patrick:

The engine will rev high and stay there easy enough with good power (that indicates to me that the ignition system and fuel supply are probably fine), but the engine will stall unless it's acceleratred very slowly.




As I say, unplugging the electrical connection of the MAP results in the engine appearing to run "normal". The throttle can be quickly cracked open and the engine will accelerate fine.

Okay, I've got to go. Thanks everyone for the suggestions so far. I'll respond further later today.

[This message has been edited by Patrick (edited 01-28-2014).]

carbon JAN 27, 03:04 PM
If you're checking voltages be sure to check TPS as well... the throttle position is used by the ECM for accel enrichment. It just pops into my head when you say you can slowly reach any RPM and it runs fine once it gets there.