90 Merc Grand Marquis/ Misses and hisses - Ford Forums

lucid · 04-19-2005, 15:12

I need some serious help with my Grand Marquis (1990 5.0 EFI ), if any one could help it would greatly be appreciated. I am so used to working on Chevys that this Ford is giving me trouble. It misses immediately at startup, and smoothes out a little when it warms up, but it has low power and misses and trembles all over. It will not idle, we have to step the idle screw way up to get it to idle. I have replaced the Air Bypass Valve, the TFI module, plug wires, Bosch Platinums, and the cap and rotor. So im thinking the computer is making some decisions on the timing that are incorrect. I have set the base timing to around 14 degrees static, with the SPOUT disconnected. Disconnected the battery and allowed it to relearn.
I pulled the codes and corrected the EGR code 32 i believe, (cleaned out the EGR valve and it disappeared) and then i got a SPOUT trouble code of 18 a couple times. The computer also gave a code stating that it could not control the idle speed during the self test (hence the new BYP valve). There is a signal going through the yellow/green SPOUT wire at the distributor, but the signal must be off. I have a Ford Fuel Injection repair manual for the EFI system and its stating that the square wave on the PIP and SPOUT at idle should be nearly identical. I dont have an oscilliscope, but when i hook my dwell meter up on the 8 cyclinder scale the dwell is definitely not the same. Also there is this loud hissing noise coming from somewhere, it is possibly a vacuum leak but i cannot find it. I torqued down the upper plenum. The vacuum meter is showing about 13 on the gauge, which seems to be pretty low. Anyone have any ideas?

Brian

BlueSteel · 04-20-2005, 12:20

Well, if its sucking huge air then the computer won't beable to calculate for the extra air in the chamber at combustion witch could cause lose of power and stumbling. On the underside of the upper intake theres a large vacuum hose, it connects to another vacuum on the back of the block, and is the vacuum assist for the power brakes. Check to see if there rotted and cracked. Another thing is the timing chain may have warn out, they use nylon gears 1984-1991, in 1992 they used single rollers full metal. In 1986 they put crank triggers on then for the EFI. If the timing chain has large amounts of slope then the computer cannot fire the injectors and spark plugs at the right time. If the car has between 70,000-80,000 miles on it then its the timing chain, a way to try to counter this slope is put the car in nuetral, let it idle with the timing advance plug unpluged (or as its called the spout) and the dist. loose, turn the dist. to 10 degrees below top dead center, then retard it to 12 degrees BTC, if that doesn't help go 8 degrees BTC, if that still does nothing, then time for a new timing chaing.

lucid · 04-21-2005, 01:17

Thanks BlueSteel, i appreciate the help, i'll give it a try tomorrow. This engine was rebuilt about a year or two ago, so i assumed the engine builder had replaced the timing belt. We shall see we shall see, this thing had a problem with the PVC system before and an oiling problem. I think that was pretty common with these Mercs. I'll update when i find out whats what.

lucid · 04-27-2005, 09:02

Just for fun, while i was looking for the source of the hissing around the throttle body, i pierced the wiring at the Throttle Position Sensor and found a 1 Volt signal with no load with the key on engine off. This seems a little high, considering i believe the idle voltage should be around 0.39 Volts. I know a voltage this high on my TPI Camaro would throw off the timing and fuel delivery. When i started the car and watched the voltage the reading started to jump around and the engine bucked and struggled to run. Do you think that this TPS is bad?

BlueSteel · 04-27-2005, 21:12

Its possible but mine is at 1.01v during start and warm up, then hits 0.86v at normal running. The brain will vary the volts and kick the throttle up if the distributor crank trigger bearing is warn and bounces. It tries to compensate, mine (I really didn't think it could but) it opened the throttle valve to let more air in to compinsate for when it had a stretched timing chain. It thought it needed more air to raise the idle and to not stall. I think it used the cruise control or something, scared the crap out of me.

Another thing you can clean to see if it fixes it is the Idle Air Bypass Sensor, its the long cylender on the throttle body that points down, a new one is about $80, just take it off, spray with carb cleaner or so, let it sook then spray with some lube, also replace the O-ring. That might solve the problem but I dought it.