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Hesitation and hard start

M

menos

NAXJA Forum User
Location
Moore, OK
So I had some problems at Superlift this weekend.
Problem is thus....
After a cold soak the truck will start right up and seem to be fine. If revved there will be a small amount of hesitation right as you get into the throttle. If the engine is warmed up the hesitation on throttle up gets way more pronounced. If the engine is shut off, restarting is a pain. 30-45 seconds of cranking with the pedal to the floor just to get it fired. Must hold 50% throttle until it evens out otherwise it will just die. The truck will run down the highway just fine and seems to have plenty of power, the problems rears its head when you attempt to get on the throttle from an idle (stop sign, etc). It's way more pronounced when you're in 4L and pointed up a 60 degree climb :( Also it is spitting black junk out of the tailpipe, especially after a hard restart.

The following was swapped in the parking lot at Superlift with no appreciable effect.
-TPS
-Coil
-Plugs
-Fuel filter

The fuel pump was replaced two weeks ago for a different problem. I threw a fuel pressure gauge on it today and it was reading about 40 under load even while hesitating and failing to start.

Any ideas?
 
Renix, OBDI or OBDII?

In other words, year/engine/trans, etc.?
 
Probably the MAP or MAT sensors are worn/damaged--grab a couple from the junkyard and see if they fix it, and if so buy good parts somewhere. Could also be weak fuel delivery, if you want to strap on a fuel pressure gauge and monitor it.
 
"black junk" would be soot (semi-burned fuel) from a very rich mixture or inconsistent spark. I'd say, since it runs decent at speed and you've replaced the plugs and coil (cap, rotor, and wires wouldn't hurt) you're probably looking for causes of a too-rich mixture.

Inspect the vacuum line from the intake to the MAP sensor first and verify it's intact and attached. After that, crawl under and verify the O2 sensor didn't get unplugged/damaged. A DMM and some time will find the culprit if everything is still hooked up, but if the symptoms came on abruptly, I'd begin by looking for rapid causes (disconnected/damaged items) first.
 
glad to hear you made it back alright-i didnt want to send you a message about it running alright and then have it die on ya ha!
can you post a picture of your spark plugs-i've been searching all over and cant seem to find a picture on the internet of what yours looked like they might tell the story to someone on here
 
I'll have to take a picture of them when I get home. Until then...
When I pulled the plugs they looked really weird. One half of the electrode was black with soot and the other half was perfectly clear, white porcelain.

Cap and rotor are new from when I changed the headgasket 3 weeks ago.
 
Mento, please let me know it you solve this problem. I'm having this same issue. First I had a vacuum leak fixed that. then my TPS was causing a high idle replaced TPS fixed that problem now I have a hard start and it I bump the throttle it hesitates just like yours. thanks PM me if to resolve this issue.
 
well-i havent heard anything but got a call about the spark plugs being half rich/half lean and was told that it could be caused by a faulty map sensor which could cause the injectors to run rich or lean when engine was hot.
 
The map sensor was my gonna be my next attempt. I hate to keep buying sensors and parts for it since if I'm out the money even if the part doesn't fix it.
Anybody know of a diagnostic test for the MAP sensor? I'll pull the multimeter out and test it but I don't know the specs.
 
So I swapped the MAP sensor. No change. Then I got to thinking about what affects how rich the engine runs. I discconnected the o2 sensor. No change. Then I disconnected the ecu temp sender. Wow! Runs great! So tomorrow I'm gonna go get another temp sender and go from there.
 
Wouldn't effect the gauge. On the '91 the gauge is driven from the sender at the back of the head. The one in the thermostat housing only drives the ECU. I'm gonna go pick one up this afternoon after work ($18) and make sure that fixes it. Wish I'd tried that before everything else. At least now I've got a spare TPS, MAP sensor and coil for the trail box.
 
Please let me know if this fixes your problem. I have the same issue. I replaced my sensor about two years ago. I wonder if its gone south.
 
So far so good. Throttle response off idle is snappier than it's been in awhile.

How come it's always the cheapest part you replace last that fixes the problem? :)
 
You replaced the one at the thermostat housing right? I guess Ill have to go grab one. thanks.
 
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