This pertains to a '93 F150 4.9 with 400+K miles. Had been still running great when all of a sudden it decided to blubber at an idle, required more throttle to get going when I let out on the clutch and intermittent "bucking" with steady throttle. I can pull up to my gate at home, get out of the truck to unlock the gate and it is idling fine. Then it is like a switch flipped and it begins its blubbering. It will also do this when driving extended times with steady throttle position at speed (55-65mph). It has not shown any change in fuel economy for the 2 weeks this has been happening daily. I did replace plugs, cap, rotor, and wires. CEL stays on, goes out intermittantly at higher rpms before shifting to the next gear (5spd. manual).

Now, I take it to a local shop and had him put his diagnostic on it. It showed MAP sensor low range but working. EGR working fine. Fuel pressure and regulator fine. Vacuum in range for an engine with this many miles. NO signal from the O2 sensor. And when revved up from an idle the timing retarded instead of advanced (??). He states that OBD1 systems are a pain in the a** to pinpoint problems. Thus, he states I should replace either the MAP sensor or the ECM.

With all this being said, can a MAP sensor do this intermittant thing (8-10 times minimum within a 45 min. drive to or from work) or does a MAP sensor fail totally without causing reocurring issues? Or could a coolant sensor cause such issues including ignition timing?

Thanks for any assistance y'all can lend my way. I'm just about to throw in the towel and pump $$ into my '94 Ranger which has issues of its own.
-Alan-

 

Hmm. I don't know your mechanic and I'm in no position to criticize his diagnosis, but when you pay someone for an opinion and he suggests your vehicle is a PIA to diagnose, I'd say that is a big red flag.

More to the point, yes, any sensor, including a MAP can show intermittent symptoms, but just a guess here, I think the statement about no O2 sensor signal is far more important. Throwing parts at the problem, (and that would include your Ranger as well), is not the answer. Replacing the MAP sensor could be advisable given that there is evidence of it being out of range, but you are a long way from needing an ECM. Sounds like another opinion is needed.

 

I would start with the O2 sensor first. The engine needs that information to run correctly.

 

Before trusting that half-assed diagnosis and replacing parts, pull the codes yourself. List them here by number and during which test they were displayed. Start with the Key On Engine Off ( KOEO ) tests and Continuous Memory ( CM ) display.

The MAP sensor has a big effect on engine operation. Much more than an O2 sensor. But I would not spend money on parts based on my opinion your mechanic's apparent lack of knowledge with OBD-I vehicles.

 

MAP sensors are cheap. ECMs, not. The 2 times I got that code, it was a hard failure & not intermittent. The first I replaced with a parts store unit & it lasted a year. The second time I put a used Motorcraft unit from the junk yard. I believe a low temp reading could pull timing along with no O2 sensor reading (Don't want to cause detonation)

I agree with Randy above.^^^

 

no o2 signal will cause limp mode. meaning open loop which will cause the fuel and timing tables to skew toward rich and retard.